CEMENT,    CONCRETE    AND    BRICKS 


OUTLINES  OF  INDUSTRIAL 
CHEMISTRY. 

A  SERIES  OF  TEXT-BOOKS  INTRO- 
DUCTORY TO  THE  CHEMISTRY 
OF  THE  NATIONAL 
INDUSTRIES. 

EDITED  BY 
GUY  D.  BENGOUGH,  M.A.,  D.Sc. 


Disintegrator  for  Coarse  Grinding. 


Mill  for  Fine  Grinding.    (Gebr.  Pfeiffer. 


Frontispiece.] 


OUTLINES   OF   INDUSTRIAL    CHEMISTRY. 

CEMENT,     CONCRETE 
AND     BRICKS 


ALFRED   B.    SEARLE 

LECTURER  ON  BRICKMAKING  UNDER  THE  CANTOR  BEQUEST  ;    CONSULTING  EXPERT 
IN  THE  CEMENT  AND  CLAY  PRODUCTS  INDUSTRIES 

AUTHOR  OF  "BRITISH  CLAYS,  SHALES  AND  SANDS,"  "THE  NATURAL  HISTORY  OF  CLAY, 

"MODERN    BRICKMAKING,"    "THE    CLAYWORKER'S    HANDBOOK"; 

JOINT    AUTHOR    OF    "  REINFORCED  CONCRETE,"  ETC. 


NEW    YORK 

D.  VAN    NOSTRAND  CO. 

TWENTY-FIVE     PARK     PLACE 
1914 


\ 


PREFACE 

OF  all  building  materials  which  are  not  found  in  a  state  of 
nature,  the  most  important  are,  unquestionably,  cement, 
concrete  and  bricks.  The  first  of  these  includes  a  large  variety 
of  materials  used  to  bind  together  particles  of  stone,  sand,  and 
other  naturally  occurring  materials  ;  the  second — used  in  a 
broad  sense — includes  all  kinds  of  artificial  stones  made  by 
cementing  various  materials  together  without  the  aid  of  heat, 
and  the  last — bricks — includes  an  even  larger  number  of 
different  articles,  distinguished  by  their  general  form  and  by 
the  fact  that  heat  has  been  used  to  render  their  shape  per- 
manent. It  is  a  mistake  to  suppose  that  all  bricks  are  made 
of  clay  ;  at  the  present  time  they  are  made  from  a  number  of 
other  materials,  such  as  destructor  refuse,  sand,  slag,  etc. 
Indeed,  the  composition  of  some  bricks  so  closely  resembles 
that  of  concrete  as  to  render  it  necessary  to  include  them  as 
one  of  the  forms  taken  by  this  material. 

It  is  important,  in  considering  the  chemical  and  other 
properties  of  these  three  typical  building  materials,  to  observe 
the  genetic  relationships  between  them.  To  neglect  this  is  to 
enter  upon  a  course  of  study  which  is  exceptionally  difficult, 
and  to  follow  a  pathway  of  thought  along  which  many  men 
have  lost  their  way.  So  long  as  cement  and  concrete  are 
considered  as  having  nothing  in  common  with  bricks,  and  vice 
versa,  it  is  almost  impossible  to  understand  the  constitution 
of  any  of  these  materials.  Separately,  they  lead  to  no 
important  conclusions,  but  considered  together  they  throw  a 
light  on  each  other's  characteristics  which  is  as  important  as 
it  is  unexpected. 

Until  a  few  years  ago  the  brick  industry  of  this  country  had 
no  men  of  sufficient  scientific  training  to  study  adequately  the 
constitution  of  the  materials  used  or  the  chemical  and  physical 
changes  which  occur  during  manufacture.  Consequently,  the 


293076 


viii  PREFACE 

industry  was  largely  worked  by  "  rule  of  thumb,"  and  men  had 
often  to  pay  very  dearly  for  their  experience,  simply  because 
there  was  no  source  from  which  to  obtain  guidance  on  the 
complex  technical  problems  associated  with  their  work. 

The  manufacturers  of  cement  have  been  more  fortunate,  for 
they  realised  at  an  early  stage  that  success  or  failure  depended 
largely  on  maintaining  a  mixture  of  constant  chemical  com- 
position ;  they  found  that  tests  were  necessary  at  so  many 
stages  in  the  manufacture  that  the  employment  of  several 
chemists  became  essential.  With  this  scientific  assistance  the 
chemical  and  physical  laws  affecting  the  production  of  cement 
were  studied  with  very  gratifying  results,  and  though  much 
remains  to  be  done,  the  importance  of  a  knowledge  of  chemistry 
in  the  manufacture  of  cement  has  been  fully  established. 

Concrete  is  in  an  intermediate  stage  so  far  as  the  application 
of  science  to  its  production  is  concerned.  It  does  not  appear 
to  lend  itself  to  such  definite  study  as  cement,  on  the  one  hand, 
or  the  changes  produced  by  heat  in  brick  materials  on  the 
other,  yet  the  physical  properties  of  concrete,  and  the  light  it 
throws  on  many  of  the  problems  met  with  in  the  manufacture 
of  bricks,  are  sufficiently  important  to  render  its  study 
invaluable,  whilst  the  increasing  use  of  concrete  by  engineers 
and  builders  renders  a  thorough  knowledge  of  its  charac- 
teristics, under  various  circumstances,  quite  essential. 

With  these  facts  in  mind  the  purpose  of  the  present  volume 
is  to  show  the  intimate  relationships  which  exist  between 
cement  and  bricks  (with  concrete  as  a  relative  of  both),  and  at 
the  same  time  to  indicate  some  of  the  directions  in  which  a 
further  study  of  any  of  these  materials  will  be  likely  to  prove 
of  value.  In  carrying  out  this  work  it  may  be  necessary,  at 
times,  to  question  statements  generally  regarded  as  facts,  and 
to  show  that  some  of  these  are  erroneous  ;  at  the  same  time, 
the  position  taken  by  the  author  is  one  of  progressive  con- 
servatism, in  which  the  old  ideas  are  retained  as  long  as  is 
reasonably  possible,  and  are  only  abandoned  where  the  evidence 
against  them  is  conclusive. 

For  the  use  of  illustrations  the  author  is  indebted  to  the 
various  firms  whose  names  are  mentioned  in  connection  with 
the  systems  of  reinforcement  described,  to  Messrs.  J.  Whitehead 


PREFACE  ix 

&  Co.,  Ltd.,  of  Preston  ;  Whittaker  &  Co.,  Ltd.,  of  Accrington  ; 
Sutcliffe,  Speakman  &  Co.,  Ltd.,  of  Leigh;  Geb.  Pfeiffer,  of 
Kaiserslautern ;  the  Associated  Portland  Cement  Manufac- 
turers, Ltd.,  the  British  Engineering  Standards  Committee,  and 
to  numerous  other  writers  and  firms  duly  acknowledged  in  the 
text.  For  much  assistance  in  writing,  and  also  in  revising  the 
proofs,  the  author  is  also  indebted  to  Mr.  J.  W.  Merchant  and 
other  members  of  his  staff. 

ALFRED    B.    SEARLE. 

THE  WHITE  BUILDING, 
-  SHEFFIELD. 

March,  1913. 


XI 


CONTENTS 


CHAP.  PAGE 

PREFACE vii 

I.     THE  RAW  MATERIALS  FOR  CEMENTS  1 

II.     METHODS  OF  CEMENT  MANUFACTURE       ....  20 

III.     THE  CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS     .  39 

IV.    THE     CHANGES     WHICH     OCCUR     IN      SETTING      AND 

HARDENING 81 

V.     TESTING  THE  PROPERTIES  OF  CEMENTS  ....  96 

VI.     THE  COMPONENTS  OF  CONCRETE  AND  THEIR  PROPERTIES  146 
VII.     THE  PREPARATION  OF  CONCRETE    .        .        .        .        .162 

VIII.     REINFORCED  CONCRETE 206 

IX.     SPECIAL  PROPERTIES  OF  CONCRETE         .        .        .        .  254 

X.     TESTING  CONCRETE .        .277 

XI.    THE  RAW  MATERIALS  FOR  BRICKS 236 

XII.     METHODS  OF  BRICKMAKING      .        .        .        .        .        .  319 

XIII.     THE  CHEMICAL  AND   OTHER  CHANGES  IN  DRYING  AND 

BURNING  BRICKS       ...                 ,  363 

XIV.     THE  PROPERTIES  OF  BRICKS 376 

XV.     SILICEOUS  BRICKS 395 

XVI.     BASIC  AND  NEUTRAL  BRICKS 401 

INDEX       .  403 


CEMENT,    CONCRETE     AND 

BRICKS 


CHAPTER  I 

THE    RAW   MATERIALS    FOR    CEMENTS 

IN  the  present  volume,  the  term  cement  is  used  with  reference 
to  those  materials  which  are  employed  to  effect  adhesion 
between  stones,  sand  and  other  aggregates  used  in  the  con- 
struction of  buildings,  reservoirs,  etc.  Cements  of  organic 
origin,  such  as  fish  glue,  seccotine,  etc.,  are  purposely  excluded, 
together  with  those  cements  (Keene's  cement,  Parian  cement, 
etc.)  which  have  gypsum  as  their  chief  constituent,  and  the 
cements  composed  of  oxy-chlorides  and  oxy-phosphates 
(Porcelain  cements). 

In  the  limited  sense  in  which  the  term  cement  is  used 
in  the  following  pages,  it  refers  to  a  variety  of  substances 
composed  essentially  of  clay  and  lime,  and  includes  Portland 
cement,  Roman  cement,  trass,  hydraulic  lime,  pozzolana,  etc. 
With  the  exception  of  the  first-named,  all  these  cements  are 
made  from  naturally  occurring  materials  without  any  ad- 
mixture, and  no  serious  attempt  is  made  to  ensure  their  having 
any  fixed  composition  or  more  than  roughly  uniform  properties. 
Portland  cement,  on  the  contrary,  is  made  by  carefully  mixing 
clay  with  limestone  or  chalk,  or  materials  corresponding  to 
these  substances,  such  as  marls  and  loams,  in  definite  propor- 
tions so  as  to  secure  a  product  of  perfectly  uniform  composition 
and  properties,  and  special  care  is  taken  by  analysis  and  other 
methods  of  testing  to  ensure  this  uniformity  in  the  product. 
This  is  one  reason  why  the  manufacture  of  Portland  cement 
now  exceeds  in  importance  that  of  all  other  hydraulic  cements  : 
the  product  is  uniform  instead  of  variable  in  its  properties, 
it  is  greatly  superior  because  the  defects  due  to  unsuitable 

c.  B 


2  THE   BAW   MATERIALS   FOR    CEMENTS 

composition  are  removed,  and,  being  made  from  materials 
which  occur  in  extremely  large  quantities  in  several  important 
industrial  areas,  its  use  enables  an  artificial  product  of  known 
composition  and  properties  to  be  used  in  place  of  an  inferior 
cement  made  from  natural  materials,  which  can  never  be  so 
reliable. 

Cements  of  the  kinds  herein  considered  are  conveniently 
termed  hydraulic  cements,  as  their  hardness,  when  set,  is  not 
appreciably  affected  by  immersion  in  water.  Zulkowsky  has 
suggested  the  term  hydraulite  (with  the  corresponding  adjective, 
hydraulic)  as  convenient  for  all  cements  of  this  class. 

There  are  no  naturally  occurring  cements,  though  some 
metamorphosed  volcanic  lavas  may  be  regarded  as  feeble 
cements.  Consequently,  cements  are  all  prepared  by  various 
methods  of  treatment. 

The  raw  materials  used  in  the  manufacture  of  cements  are 
clays,  chalk,  limestone,  volcanic  tuifs  and  lavas,  various  muds 
more  or  less  related  to  these  substances  and  sundry  by-products 
of  other  industries,  as  slags.  These  raw  materials  may  be 
classified  as  follows  : — 

(a)  Those  which  require  to  be  mixed  with  one  or  more 
substances  and  then  heated  and  ground  before  a  cement  is 
obtained  (clays  and  limestones). 

(b)  Those  which  merely  require  to  be  heated  and  ground 
in  order  to  form  a  cement  (natural  cements  and  hydraulic  limes). 

(c)  Those  which  require  no  heating,  but  must  be  mixed  with 
a  complementary  substance,  such  as  lime,  before  a  cement  is 
produced  (pozzolanas) . 

(d)  Those  which  merely  require  to  be  ground  (slags). 

(a)  The  Raw  Materials  o/  Portland  Cements. 
The  raw  materials  used  in  the  manufacture  of  Portland 
cement  are  essentially  lime  and  clay — a  compound  of  these 
two  substances,  in  which  the  lime  plays  the  part  of  a  base 
and  the  clay  that  of  an  acid,  being  formed  when  they  are 
heated  together  to  a  sufficiently  high  temperature.  As 
combination  does  not  take  place  until  a  temperature  is  reached 
which  is  higher  than  that  at  which  calcium  carbonate  is 
converted  into  quicklime,  it  is  customary  to  use  some  naturally 


CHALK 


occurring  form  of  calcium  carbonate,   such  as  limestone  or 
chalk,  in  place  of  lime  itself. 

The  lime  compounds  used  in  the  manufacture  of  hydraulic 
cements  furnish  the  basic  portion  of  the  product.  It  is  essential 
that  they  shall  produce  quicklime  at  a  temperature  lower  than 
that  at  which  the  cement-clinker  is  produced,  and  for  this 
reason  only  the  naturally  occurring  calcium  carbonates  are 
employed. 

LIME  COMPOUNDS. 

The  chief  lime  compounds  used  in  the  manufacture  of 
Portland  cement  are  as  follows  :— 

Chalk  is  a  soft  limestone,  and  is  composed  mainly  of  minute 
sea-shells,  but  these  have  been  so  damaged  by  earth-move- 
ments, etc.,  that  it  is  only  occasionally  that  their  definite 
structure  can  be  recognised.  As  found  in  the  south-eastern 
counties,  chalk  is  a  fairly  pure  form  of  calcium  carbonate, 
which  occurs  as  white,  compact  masses  which  are  readily 
crushed  to  powder.  Flints  and  other  stones  and  impurities 
occur  in  the  chalk,  and  these  should  be  removed  by  stirring 
the  chalk  with  water  in  a  wash-mill,  the  creamy  white  fluid 
so  obtained  being  run  off  and  the  impurities  remaining  behind 
in  the  mill. 

Chalk  is  largely  used  in  Kent  and  Sussex,  where  it  occurs, 
and  considerable  quantities  of  it  are  sent  to  Northumberland 
and  Yorkshire,  where  it  is  used  in  connection  with  a  local 
clay. 

ANALYSES  OF  CHALK  (after  drying  at  110°  C.). 


Medway 

Gravesend 

Purfleet 

Ilarefleld 

Kitchen 

Cambridge 

Upper 

Upper 

Upper 

Upper 

Middle 

Lower 

Chalk. 

Chalk. 

Chalk. 

Chalk. 

Chalk. 

Chalk. 

Silica      .... 

2-34 

1-12 

1-22 

1-86 

0-69 

3-57 

Alumina 

1-84 

0-21 

0-58 

0-63 

0-57 

0-56 

Iron  oxide 

1-49 

0-13 

0-11 

0-30 

0-37 

0-14 

Calcium  carbonate  . 

92-18 

97-88 

97-82 

97-01 

96-81 

91-94 

Magnesium  carbonate 

1-16 

0-41 

0-25 

0-15 

1-42 

1-37 

Other  substances 

0-99 

0-25 

0-02 

0-05 

0-14 

2-42 

Wherever  chalk  and  a  suitable  clay  are  found  in  sufficiently 
close   association,    Portland   cement   may   be   manufactured. 

B2 


4  THE   RAW  MATERIALS    FOR    CEMENTS 

Where  the  two  materials  do  not  occur  together,  the  nature  of 
the  demand  for  the  cement  will  largely  determine  the  most 
suitable  site  for  the  works.  In  the  south-eastern  counties  the 
works  are  close  to  the  chalk,  and  the  clay  is  brought  to  them 
from  some  distance  ;  at  Newcastle  and  Hull,  on  the  contrary, 
the  works  are  situated  on  the  clay  and  the  chalk  is  brought 
from  the  south-eastern  counties. 

Limestone,  like  chalk,  consists  chiefly  of  calcium  carbonate  ; 
it  has  been  coloured  by  trifling  proportions  of  organic  matter. 
It  is  much  harder  than  chalk,  and  must  be  crushed  before  use. 
This  makes  its  use  more  costly  than  that  of  chalk. 

The  limestone  chiefly  used  for  cement  manufacture  is  of 
Liassic  origin,  and  occurs  in  the  Midlands  and  South  Wales 
in  association  with  shale.  Limestones  which  are  free  from 
shale  or  clay  are  inconvenient  because  of  the  cost  of  bringing 
the  two  essential  materials  together ;  hence,  argillaceous 
limestones  are  almost  invariably  selected. 

Some  oolitic  limestones  are  used,  notably  in  Northamp- 
tonshire. 

Liassic  limestones  are  characterised  by  great  regularity  in 
composition  when  sufficiently  large  quantities  are  examined  ; 
smaller  quantities  show  considerable  variations  in  the  different 
strata,  numerous  bands  of  clay  or  shale  alternating  with  those 
of  limestone.  In  some  quarries  thirty  or  more  different  beds^ 
may  clearly  be  recognised. 

The  following  analyses  are  typical  of  the  Lower  Lias  lime- 
stones used  for  cement  making  : — 


ANALYSES  OF  LIAS  LIMESTONES  (after  drying  at  110°  C.). 


Warwick- 
shire. 

Shropshire. 

South 
Wales. 

Northamp- 
tonshire. 

Silica       .... 

13-25 

8-43 

1-28 

6-85 

Alumina 

5-72 

5-27 

0-51 

2-74 

Iron  oxide 

1'97 

1-76 

1-42 

2-85 

Calcium  carbonate    . 

77-46 

81-82 

94-76 

86-12 

Magnesium  carbonate 

1-35 

2-47 

1-62 

0-67 

Other  substances 

0-25 

0*25 

0-41 

0-77 

LIME   COMPOUNDS   AND    CLAYS  5 

Great  care  must  be  taken  not  to  use  dolomitic  limestone 
which  contains  much  magnesia,  as  this  substance  is  deleterious 
to  the  resultant  cement. 

Argillaceous  limestones  are  the  raw  material  from  which 
hydraulic  limes  are  made  (p.  13). 

If  the  limestone  is  sufficiently  free  from  clay  or  shale  it 
will,  when  burned,  produce  quicklime.  If  the  latter  is  slaked 
and  then  ground  with  sand  and  water  it  forms  ordinary 
builders'  mortar.  The  quality  of  such  mortar  depends  on  the 
purity  of  the  limestone,  satisfactory  burning,  and  thorough 
mixing  or  tempering  with  sand.  Many  builders  at  the  present 
time  do  not  pay  sufficient  attention  to  these  details  and  so 
obtain  a  mortar  of  inferior  quality. 


CLAYS,  SHALES,  MARLS  AND  MUDS. 

The  clays  and  similar  materials  used  in  the  manufacture  of 
Portland  cement  furnish  the  acid  constituent  which  combines 
with  the  lime  to  form  the  final  product.  The  essential  con- 
stituent of  all  clays  is  aluminosilicic  acid,  but  whether  there 
is  one  or  more  varieties  of  this  substance  is,  at  present, 
uncertain.  In  the  author's  opinion  there  are  many  alumino- 
silicic acids  which  have  so  similar  a  chemical  composition  that 
some  of  them  cannot  readily  be  distinguished  by  analysis. 
Such  compounds  are  known  as  isomers  ;  they  differ  greatly  in 
many  ways  from  each  other,  especially  in  physical  properties. 
Many  clays  used  in  cement  manufacture  are  probably  isomeric, 
their  apparent  similarity  in  analysis  being  merely  a  coincidence. 
The  study  of  isomers  forms  an  important  branch  of  organic 
chemistry  ;  but  little  attention  has,  as  yet,  been  paid  to  complex 
inorganic  isomeric  substances.  (See  Chapter  III.). 

Even  if  several  distinct  aluminosilicic  acids  should  be 
found  to  be  the  essential  constituents  of  various  clays,  their 
composition  and  properties  must  be  very  similar,  as  the 
product  obtained  by  refining  most,  if  not  all  clays  has  a  com- 
position corresponding  to  the  formula  xH20  .  yAl^O^  .  zSiO2. 
Thus,  carefully  washed  china  clay,  which  is  the  purest  clay 
known,  is  composed  of  ;— 


THE   RAW  MATERIALS    FOR  CEMENTS 

Alumina          .          .  39  '45 

Silica      ....  46-64 

.Water  13' 91 


or  HiSi2Al2O9.  lOO'OO 

Together  with  the  aluminosilicic  acid,  clays  contain  very 
variable  proportions  of  impurities,  the  chief  of  which  are  free 
silica,  iron  oxide  and  sulphide,  lime  and  magnesia  compounds, 
together  with  various  minerals  such  as  mica,  felspar  and  other 
siliceous  rocks. 

The  clays  chiefly  used  for  the  manufacture  of  Portland 
cement  are  those  commonly  known  as  surface  clays.  Their 
composition  is  unimportant  providing  that  they  do  not  contain 
an  excessive  amount  of  coarse  sand.  Lean  or  slightly  plastic 
clays  are  better  than  those  of  a  more  sticky  character,  as  they 
are  more  easily  treated  ;  the  highly  plastic  clays  are  usually 
somewhat  richer  in  aluminosilicic  acid. 

It  is  only  natural  that  those  clays  should  chiefly  be  used 
which  are  situated,  in  convenient  localities,  and  at  the  present 
time  the  gault  clay  of  Sussex,  a  surface  clay  in  Northumberland 
of  apparently  glacial  origin,  and  an  alluvial  clay  at  Hull  are 
the  ones  most  extensively  employed.  In  other  localities  more 
conveniently  situated,  clays  are  employed  on  a  small  scale. 
Thus,  there  is  no  reason,  except  lack  of  demand  for  cement  in  the 
localities  where  they  occur,  why  the  extensive  deposits  of  Triassic 
clays  should  not  be  more  largely  used  than  is  at  present  the  case. 

The  Gault  clay  occurs  in  the  lower  portion  of  the  chalk 
formation,  immediately  above  the  Greensand  or  between  the 
upper  and  lower  Greensand.  It  is  a  very  stiff,  dark  blue  or 
black  clay,  which  often  becomes  brown  on  weathering.  It 
extends  from  Folkestone  on  the  east  to  Eastbourne  on  the 
west,  and  also  occurs  in  Norfolk,  Cambridgeshire,  Hertford- 
shire, Bedfordshire,  Buckinghamshire,  and  to  a  small  extent 
on  the  east  coast  of  Yorkshire,  near  Filey.  For  cement 
making  it  is  chiefly  used  in  Sussex  and  Kent.  It  contains  a 
considerable  amount  of  calcium  carbonate  (chalk) — occasionally 
as  much  as  33  per  cent,  and  seldom  less  than  13  per  cent.— 
together  with  nodules  of  pyrites  and  calcium  phosphate. 
These  nodules  should  be  removed  by  washing  the  clay. 


CLAYS,    SHALES,   MARLS    AND   MUDS 


The  composition  of  gault  clay  varies  considerably  ;  the 
following  shows  the  composition  of  samples  taken  from  the 
upper  reaches  of  the  Medway  : — 

Silica 44-23 

Alumina        .....  14*85 

Iron  oxide     .          .          .          .          .  5 '93 

Calcium  carbonate           .          .          .  26 '54 

Magnesium  carbonate     .          .          .  6 '06 

Alkalies 0'51 

Water,  etc 1-88 

Alluvial  clays  are  found  on  the  sites  of  former  lakes  or  river 
beds  and  near  to  the  banks  of  existing  rivers.  They  occur 
in  various  parts  of  the  country,  particularly  in  East  Yorkshire, 
Lincolnshire,  Cambridgeshire,  Norfolk,  Suffolk,  Essex,  Kent, 
Sussex,  Somerset  and  Lancashire,  and  to  an  even  larger  extent 
in  central  Ireland.  Small  areas  occur  in  other  low-lying 
districts.  These  clays  have  usually  been  deposited  under 
such  conditions  that  they  are  relatively  light  and  somewhat 
open  in  texture,  though  some  of  them  are  highly  plastic. 
They  are  seldom  free  from  appreciable  proportions  of  very 
finely  divided  calcium  carbonate,  which  is  an  advantage  to 
the  cement  manufacturer,  but  is  objectionable  to  some  brick- 
makers.  As  the  term  "  alluvial  "  implies,  these  clays  have 
been  collected  from  a  variety  of  sources  and  their  composition 
varies  in  each  locality  in  which  they  occur.  The  chief  alluvial 
clays  used  for  cement  making  show  the  following  composition 
on  analysis  :— 

ALLUVIAL  CLAYS  AND  MUDS  (after  drying  at  110°  C.). 


Hull. 

Thames  Estuary  Mud  . 
a.                      b. 

Medway  Mud. 

Silica       .... 

51-32 

60-12 

73-68 

51-98 

Alumina 

20-76 

10-81 

7-46 

15-63 

Iron  oxide 

6-89 

7-46 

11-13 

9-12 

Calcium  carbonate    . 

10-44 

8-31 

2-36 

4'08 

Magnesium  carbonate 

7-81 

1-86 

1-61 

2-76 

Alkalies  .... 

1-86 

0-62 

1-84 

0-92 

Water,  etc. 

0-92 

10-82 

1-92 

15-51 

THE   RAW  MATERIALS  FOR    CEMENTS 


Alluvial  clays  are  essentially  superficial  in  character  and 
may  be  distinguished  from  the  other  great  class  of  surface  clays 
(the  boulder  clays)  by  the  absence  of  large  stones  in  which 
the  boulder  clays  abound. 

Loams  are  naturally  occurring  mixtures  of  sand  and  clay. 
They  are  of  little  use  for  cement  manufacture  on  account  of  the 
large  proportion  of  sand  they  contain,  but  clayey  loams  are 
occasionally  used  with  advantage.  Strictly  speaking,  any  clay 
which  contains  free  silica  (sand)  is  a  loam,  but  the  term  is 
only  applied  to  those  with  a  low  plasticity  and  open  texture. 

Shales  are  naturally  indurated  clays  which  have  been  com- 
pressed to  almost  stony  hardness  by  the  forces  of  nature,  but 
the  term  is  also  applied  to  any  rock  which  can  be  split  into 
thin  layers,  so  that  some  shales  contain  very  little  clay. 

Slate  waste — a  variety  of  shale — has  been  used  in  America 
for  the  manufacture  of  cement. 

'     ANALYSES  OF  SHALES  (after  drying  at  110°  C.). 


Warwick- 
shire. 

Shropshire. 

South 
Wales. 

Northamp- 
tonshire. 

SilW      . 

31-74 

8-07 

58-17 

8-41 

Alumina 

.         '  .           .  ...)       9*23 

5-23 

14-34 

5-39 

Iron  oxide 

3-14 

1-84 

5-11 

1-72 

Calcium  carbonate    . 

47-41 

80-17 

12-61 

77-29 

Magnesium 
Alkalies  . 

carbonate 

7'35 
0-37 

3-73 

5-98 
2-47 

5'86 
0-97 

Water,  etc. 

0'76 

0-96 

1-32 

0-36 

Clay  shales  occur  in  enormous  quantities  in  Great  Britain, 
particularly  in  Coal  Measure,  Permian,  Jurassic  and  Wealden 
regions,  and  are  of  very  diverse  character  and  composition. 
Some  shales  are  so  closely  interstratified  with  limestone  that 
it  is  difficult  to  separate  these  substances,  but  this  is  not 
important  where  the  material  is  to  be  used  for  cement  manu- 
facture, providing  that  sufficient  limestone  is  available.  For 
cements,  the  sandy  and  bituminous  shales  are  of  little  value. 
Shales  are  costly  to  grind,  and  so  are  only  used  for  cement 
when  found  in  close  association  with  limestone,  the  Liassic  beds 


CLAYS,    SHALES,    MARLS    AND   MUDS  9 

of  the  Midlands  and  especially  of  Warwickshire  and  the  shales 
of  South  Wales  being  chiefly  employed  for  this  purpose. 

The  chemical  composition  of  shales  varies  at  different 
depths  in  the  deposit  as  well  as  in  different  areas,  but  if 
sufficiently  large  quantities  are  obtained  at  a  time  and  a  little 
care  is  taken  to  mix  the  strata  in  approximately  constant 
proportions,  a  sufficiently  uniform  product  may  be  obtained 
without  difficulty. 

For  brickmaking,  the  shales  must  be  free  from  large  percen- 
tages of  lime  and  pyrites,  but  for  cement  making  the  lime  is  an 
advantage.  The  pyrites  is  objectionable  in  both  cases  on 
account  of  its  discolouring  power.  The  organic  matter  present 
in  shales — sometimes  exceeding  5  per  cent. — is  an  advantage 
in  so  far  as  it  saves  fuel,  but  it  tends  to  cause  an  overheating 
of  the  material  in  the  kilns. 

Muds  are  essentially  impure  clays  which  have  not  been 
compacted  by  pressure.  Those  found  in  the  estuaries  of  the 
Thames  and  Medway  are  largely  used  for  cement  manufacture 
on  account  of  the  readiness  with  which  they  can  be  mixed 
with  washed  chalk.  Some  muds  consist  almost  exclusively  of 
lime  dust,  and  if  these  are  to  be  used  for  cement,  clay  must 
be  added  to  them  (see  Alluvial  Clays,  p.  7). 

The  composition  of  the  muds  used  varies  greatly,  and,  like 
that  of  the  clays  previously  mentioned,  is  of  little  importance 
providing  that  the  muds  contain  sufficient  real  clay.  On 
account  of  the  ease  with  which  they  can  be  converted  into  a 
slurry,  muds  are  generally  mixed  with  chalk  slurry  in  the 
manufacture  of  cement. 

Marls  are  naturally  occurring  mixtures  of  clay  and  calcium 
carbonate  (chalk),  but  the  term  is  also  used  in  a  misleading 
manner  for  friable  rocks  which  are  free  from  lime  compounds. 
The  chief  marls  used  in  the  manufacture  of  Portland  cement 
are  friable  limestones  of  fine  texture  and  lacking  the  compact- 
ness and  coherence  of  ordinary  limestones.  Cement  makers 
specially  favour  those  found  in  Cambridgeshire  between  the 
Chalk  and  the  Greensand  formations,  but  marls  from  other 
districts — such  as  those  in  the  centre  of  Ireland — may  be  used 
with  equal  advantage  when  the  demand  for  cement  in  their 
locality  is  sufficiently  great, 


10          THE   RAW  MATERIALS  FOR   CEMENTS 

Marls  have,  apparently,  been  formed  by  the  natural 
admixture  of  clayey  and  chalkey  slimes  ;  streams  carrying  the 
finely  divided  clay  or  chalk  in  suspension  meeting  at  some 
locality  and  depositing  their  burdens  in  the  form  of  a  mixture. 
There  is,  however,  some  considerable  divergence  of  opinion  as 
to  their  true  origin,  more  especially  as  many  marls  contain 
more  silica  than  corresponds  to  a  mixture  of  clay  and  chalk. 
This  additional  silica  may  be  in  the  free  state  (as  appears  most 
likely),  but  Le  Chatelier's  experiments  lend  support  to  the 
view  that  marls  contain  an  aluminosilicic  acid  which  is  richer 
in  silica  than  is  that  usually  found  in  "  clays."  If  the  mode  of 
formation  indicated  above  is  correct  it  is  clear  that  marls  in 
different  localities  must  vary  greatly  in  composition  ;  some  of 
them  being  rich  in  clay  and  poor  in  lime,  whilst  others  are  so 
poor  in  clay  as  to  be  regarded  as  impure  lime  compounds. 
For  cement  manufacture,  the  most  suitable  marls  are  those 
which  have  a  composition  resembling  that  of  raw  Portland 
cement  slurry. 

For  Portland  cement  manufacture,  the  composition  of  the 
marl  must  be  ascertained  frequently,  so  that  the  proportions 
of  the  other  ingredients  of  the  "  mix  "  may  be  accurately 
adjusted.  For  this  reason,  many  cement  manufacturers  prefer 
to  use  a  relatively  pure  chalk  on  which  they  can  rely,  rather 
than  a  marl  with  an  irregular  composition. 

It  is  very  important  that  the  marl  should  be  fine  in  texture 
and  free  from  hard  lumps,  for  pieces  the  size  of  a  pea  will 
seriously  increase  the  cost  of  manufacture,  and  if  left  in  the 
material  they  are  very  liable  to  produce  an  irregular  cement. 
Marls  differ  greatly  in  this  respect  and  great  care  is,  therefore, 
needed  in  their  selection. 

Marls  for  cement  manufacture  differ  from  those  used  for 
brickmaking  on  account  of  the  much  larger  percentage  of 
lime  permissible  in  the  former.  For  brickmaking,  marls  with 
60  per  cent,  of  calcium  carbonate  are  almost  useless,  but  for 
cement  they  are  more  suitable  than  those  containing  at  most 
12  per  cent,  of  chalk,  which  is  the  largest  proportion  acceptable 
to  a  brickmaker.  In  this  way  a  material  useless  to  one  branch 
of  clay- working  is  of  great  advantage  to  another  branch. 

Pure  marls  are  white  in  colour,  but  most  marls  are  grey  on 


CLAYS,   SHALES,    MARLS  AND   MUDS 


11 


account  of  the  organic  matter  they  contain.  This  burns  away 
on  heating,  and  is  of  no  importance  to  manufacturers. 

Marls  which  burn  red  are  seldom  useful  for  cement  manu- 
facture unless  they  are  rich  in  clay  and  sufficient  limestone  or 
chalk  is  available  in  the  same  locality. 

Marls  from  the  Permian  formation  are  usually  rich  in 
magnesia,  and  should  be  avoided  by  cement  manufacturers. 
Liassic  marls,  on  the  contrary,  are  valuable. 

The  "  Midland  Marls"  are  not  true  marls  (but  Triassic 
clays),  and  do  not  contain  enough  lime  to  be  so  named.  They 
are  greatly  prized  for  the  manufacture  of  red  bricks  and  terra 
cotta,  but  before  they  can  be  used  for  cement  manufacture 
they  must  have  nearly  three  times  their  weight  of  limestone 
or  chalk  added  to  them.  The  Cambridge  marls,  on  the 
contrary,  are  argillaceous  limestones  with  a  composition  so 
similar  to  that  of  unburned  cement  slurry  that  they  are  used 
for  natural  cements. 

ANALYSES  OF  MARLS  USED  FOR  CEMENT 
(after  drying  at  110°  C.). 


Cambridge. 

Petersfield. 

Silica 

18-16 

26-82 

Alumina 

. 

7'82 

2-14 

Iron  oxide 

. 

0-94 

3-47 

Calcium  carbonate 

68'16 

52-86 

Magnesium 

carbonate   . 

2-77 

1-49 

Water,  etc. 

. 

2-15 

13-22 

Broadly  speaking,  the  clays  used  in  cement  manufacture 
should  be  highly  siliceous,  and  should  contain  60  to  70  per  cent, 
of  silica  and  6  to  20  per  cent,  of  alumina.  Magnesia  should 
not  exceed  3  per  cent.,  and  the  alumina  and  iron  oxide  together 
should  not  exceed  half  the  silica.  Clays  very  rich  in  iron 
should  be  avoided  as  they  produce  a  dark  coloured  cement  ; 
those  with  less  than  10  per  cent,  of  iron  oxide  are  not  objec- 
tionable in  this  respect.  Also,  clays  containing  stones  should 
be  avoided  as  they  unduly  increase  the  cost  of  grinding. 


12          THE   RAW  MATERIALS   FOR    CEMENTS 

All  clays  on  heating  to  a  dark  red  heat  are  decomposed,  the 
aluminosilicic  acid  splitting  up  into  free  silica,  alumina  and 
water,  or  else  forming  a  compound  in  which  the  silica  and 
alumina  are  exceptionally  able  to  combine  with  lime  ;  if  the 
temperature  and  proportions  of  each  of  these  substances  are 
suitable  a  clinker  is  formed  which,  on  grinding,  produces 
Portland  cement.  The  reactions  which  occur  during  the 
formation  of  this  clinker  are  described  in  Chapter  III. 

Further  particulars  of  the  occurrence  of  clays,  shales  and 
muds  suitable  for  the  manufacture  of  Portland  cement*  will 
be  found  in  the  author's  "  British  Clays,"  published  by  Messrs. 
C.  Griffin  &  Co.,  London,  W.C. 

The  proportions  in  which  the  clay  or  its  equivalent  and  the 
lime  compound  are  mixed  to  produce  Portland  cement  depends 
on  the  precise  composition  of  the  actual  materials  selected. 
The  mixture  (before  burning)  should  contain  75  to  77  per  cent, 
of  calcium  carbonate  and  23  to  25  per  cent,  of  real  clay,  the 
best  proportions  being  found  by  trial  and  adhered  to  as  closely 
as  possible  in  the  course  of  manufacture.  It  is  clear  that  if 
the  "  clay  "  contains  lime  compounds  (like  the  marls),  less 
limestone  or  chalk  will  be  needed  than  when  a  lime-free  clay 
or  shale  is  used.  The  great  value  of  Portland  cement  lies  in 
the  accuracy  with  which  the  materials  are  mixed  so  as  to  obtain 
a  product  the  composition  of  which  is  exceedingly  uniform. 
This  uniformity  can  only  be  secured  by  paying  the  closest 
attention  to  the  composition  of  the  mixture  before  it  is  burned, 
and,  in  all  cement  works  of  importance,  analyses  of  the  mixture 
are  made  daily,  or  even  more  frequently. 

(b)  The  Raw  Materials  used  in  the  Manufacture  of 
Natural  Cements  and  Hydraulic  Ijimes. 

The  materials  used  for  the  production  of  natural  cements, 
chiefly  clayey  limestones  or  highly  calcareous  marls,  are 
characterised  by  containing  calcium  carbonate  (chalk)  and 
aluminosilicic  acid  (clay)  in  proportions  similar  to  those  in 
Portland  cement.  Those  usually  employed  in  this  country 
are  in  the  form  of  calcareous  nodules  or  septaria  1  which  are 

1  Septaria  are  nodules  of  impure  limestone  which  occur  in  some  clays  and  derive 
their  name  from  characteristic  divisional  lines  (septae).  These  lines  appear  to  be 


NATURAL    CEMENTS 


13 


dredged  from  the  sea-bottom  near  Harwich,  Sheppey,  etc.,  or 
are  obtained  from  dry  land  near  the  coast,  as  was  formerly 
the  case  at  Speeton  in  Yorkshire.  Calcareous  nodules  from 
beds  of  Kimeridge,  Greensand  and  Liassic  formations,  and 
some  of  the  Cambridgeshire  marls  (p.  9)  are  also  used  for 
this  purpose. 

ANALYSES  OF  NATURAL  CEMENT  NODULES. 


Sheppey 
Septaria. 

Harwich 
Septaria. 

Speeton 
Nodules. 

Tournai 
Earth,  i 

Silica       .... 

17'84 

20-74 

20-43 

21—28 

Alumina 

6'42 

4-21 

8-81 

3—5 

Iron  oxide 

4-13 

7'85 

6-87 

1-21 

Calcium  carbonate    . 

63-76 

58-79 

56-36 

60—63 

Magnesium  carbonate 

4-37 

5-46 

3-42 

i-i  i 

Water,  etc. 

3*48 

2-95 

4'11 

,  Where  a  bed  of  rocky  material  is  of  suitable  composition 
(i.e.,  with  a  composition  similar  to  that  of  the  mixture  of  clay 
and  chalk  used  for  Portland  cement)  to  be  calcined  without 
any  admixture,  the  product  is  termed  rock  cement.  Large 
quantities  of  this  kind  of  cement  are  produced  annually  in 
America,  but  the  troubles  caused  by  irregularities  in  com- 
position have  made  many  manufacturers  use  this  material 
as  the  basis  for  Portland  cement,  the  composition  being 
adjusted  from  time  to  time  by  the  addition  of  clay  or  limestone 
as  may  be  required. 

As  the  materials  are  used  without  any  purification,  rock 
and  natural  cements  usually  suffer  from  the  presence  of  an 
excess  of  iron  oxide,  which  colours  them  dark  brown,  and  of 
inconvenient  proportions  of  magnesia,  etc. 

HYDRAULIC  LIMES. 

The  properties  of  hydraulic  lime  were  first  recognised  by 
John  Smeaton  in  1760,  who,  when  investigating  the  properties 

shrinkage  cracks,  which  have  become  filled  with  crystallised  calcium  carbonate. 
The  use  of  septaria  for  cement  manufacture  was  first  patented  by  James  Parker 
in  1796  ;  it  has  since  been  known  as  Roman  cement. 

1  The  material  of  which  Belgian  cement  (p.  31)  is  made. 


14          THE   RAW  MATERIALS   FOR   CEMENTS 

of  different  limestones  for  use  in  ths  Eddystone  lighthouse, 
found  that  the  Aberthaw  limestone  and  others  rich  in  clay  were 
more  resistant  to  immersion  in  water  than  purer  limestones. 
A  hydraulic  lime  is  one  prepared  by  calcining  an  argillaceous 
limestone,  the  clay  present  entering  into  combination  with  a 
portion  of  the  lime  and  forming  what  may  be  regarded  as  a 
mixture  of  Portland  cement  and  quicklime.  Its  value  as  a 
hydraulite  must,  therefore,  depend  on  the  extent  to  which 
the  lime  and  clay  have  combined. 

Hydraulic  limes  differ  from  ordinary  lime  in  slaking  Jess 
readily  and  in  setting  to  a  hard  stony  mass  when  immersed 
in  water. 

The  chief  raw  material  in  this  country  for  hydraulic  limes  is 
the  blue  Liassic  limestone  of  Warwickshire,  South  Wales,  etc. 
(p.  4),  but  other  argillaceous  limestones  may  be  used.  As 
will  be  understood  from  the  previous  section  on  the  raw 
materials  used  for  Portland  cement,  a  much  superior  cement 
is  obtained  when  the  composition  of  the  material  is  adjusted 
so  as  to  give  a  product  of  approximately  the  same  composition 
as  Portland  cement. 

The  composition  of  the  limestones  used  for  making  hydraulic 
limes  must  lie  between  (1)  pure  limes  which  are  free  from  clay, 
and  (2)  marls  or  mixtures  of  clay  and  chalk  which  contain  no 
excess  of  lime.  It  has  been  found  that  argillaceous  limestones 
with  70  to  80  per  cent,  of  calcium  carbonate,  10  to  17  per  cent, 
of  silica  and  not  more  than  3  per  cent,  of  iron  and  alumina  are 
best,  as,  in  the  hydraulic  limes  made  from  these,  most  of  the  clay 
is  combined  with  lime,  yet  there  is  sufficient  free  lime  present 
to  cause  the  material  to  slake  satisfactorily.  Hydraulic  limes 
may  also  be  produced  by  under-burning  a  rock  which  would, 
at  a  higher  temperature,  produce  an  excellent  natural  cement, 
but  these  are  very  inferior  and  unsatisfactory. 

(c)  Volcanic  Lavas,  Tuffs  and  Trass. 

Both  the  Greeks  and  Romans  were  aware  that  the  addition 
of  certain  materials  of  volcanic  origin,  in  a  finely  ground 
condition,  to  mortar  had  the  effect  of  making  it  hydraulic. 
Such  lavas  and  tuffs  have  already  been  heated  previous  to 


LAVAS,  TUFFS  AND  TRASS         15 

their  discharge  from  the  volcano,  and  when  mixed  with  lime 
they  form  pozzolanic  cement  or  pozzolana,  from  the  occurrence 
near  Pozzoli,  in  the  Bay  of  Naples,  of  the  most  typical  material 
of  this  kind. 

Pozzolana  is  chiefly  obtained  in  Italy,  in  south-eastern 
France,  and  in  the  Azores.  It  is  usually  found  near  the 
siirface,  but  some  of  the  Italian  workings  are  several  hundred 
feet  in  depth. 

All  the  natural  pozzolanas  are  volcanic  lavas  which  have 
undergone  subsequent  changes  (now  attributed  to  the  action 
of  superheated  steam  and  carbon  dioxide),  whereby  they  have 
been  reduced  to  a  fine  sand  and  have  gained  hydraulic  proper- 
ties. Their  composition  is  very  variable,  but  their  essential 
constituents  are  the  same  as  those  of  clays  which  have  been 
heated  to  dull  redness.  Their  value  depends  on  the  amount 
of  silica  and  alumina  (possibly  also  ferric  oxide)  present  in  a 
form  in  which  it  can  produce  hydraulitic  compounds  when 
the  pozzolana  is  mixed  with  lime  and  water.  Though  generally 
amorphous,  pozzolanas  not  infrequently  contain  crystals  of 
various  igneous  rocks. 

Artificial  pozzolanas  have  been  made  since  Roman  times  by 
heating  clays  to  redness  or,  more  frequently,  by  crushing 
burnt  clay  ballast,  broken  bricks,  or  tiles  to  a  fine  powder. 
Broken  pottery  (potsherds)  is  occasionally  used,  but  these 
should  be  of  porous  material,  not  of  vitrified  stoneware  or 
porcelain . 

The  completeness  with  which  brick  or  tile  dust  can  be  used 
to  replace  natural  pozzolana  is  strongly  confirmative  of  the 
view,  already  expressed,  that  natural  pozzolanas  and  trass 
have  the  same  composition  and  general  properties  as  calcined 
clay. 

Tosca  is  a  pozzolana  or  volcanic  ash  obtained  from  Teneriffe, 
in  the  Canary  Islands,  and  chiefly  used  in  Spain. 

Trass  is  also  a  metamorphosed  volcanic  lava,  the  most 
important  deposits  being  found  near  the  Rhine.  It  resembles 
pozzolana  in  many  ways,  and  has  a  similar  composition,  but  is 
quite  distinct  from  it. 

Santorin  earth  is  another  similar  material  obtained  from  the 
Greek  island  of  that  name.  It  is  usually  slightly  more  siliceous 


16  THE   RAW  MATERIALS   FOR    CEMENTS 

and  contains  rather  less  alumina  than  Rhenish  trass  or  true 
pozzolana  earth. 

(d)  Slags 

are  really  glasses  and  are  in  the  state  of  an  extremely  viscous 
fluid,  the  rigidity  of  which  is  apparently  equal  to  that  of  a 
solid,  though  it  is  devoid  of  any  crystalline  structure.  All 
glasses  are  in  a  state  of  instability  and  tend  to  crystallise  just- 
as  a  slowly  cooling  liquid  does,  only  far  more  slowly.  The 
conversion  into  the  more  stable  crystalline  form  is  hindered 
by  the  enormous  viscosity  of  the  material  ;  if  this  is  reduced 
(as  by  heating  the  substance  to  below  its  melting  point), 
crystallisation  proceeds  more  rapidly. 

It  is  extremely  difficult  to  ascertain  what  compounds  actually 
exist  in  granulated  slags,  as  prolonged  heating  may  cause  a 
molecular  learrangement  of  their  constituents,  whereby  the 
crystals  so  formed  would  not  represent  the  composition  of 
the  slag  out  of  which  they  grew.  In  any  case,  the  crystals 
found  in  granulated  slags  lack  the  characteristic  forms  of  the 
calcium  orthosilicate  they  most  closely  resemble  (gehlenite 
and  melilite),  and  their  identity  has  not  been  satisfactorily 
established.  The  commonly  accepted  view  is  that  granulated 
basic  slags  contain  a-dicalcium  orthosilicate,  but  this  is  based 
on  evidence  which  is  by  no  means  conclusive. 

W.  and  D.  Asch  have  assigned  to  such  slags  formulae  bearing 
a  resemblance  to  those  used  by  them  for  clays  and  cements. 

Some  slags  obtained  from  blast  furnaces  as  a  by-product  in 
the  manufacture  of  pig  iron,  contain  the  same  constituent  oxides 
as  Portland  cement,  but  not  necessarily  in  the  same  propor- 
tions. In  a  well-managed  ironworks  the  composition  of  such 
slags  is  very  constant,  and,  as  they  are  of  small  value  and 
require  a  large  amount  of  storage  space,  many  attempts  have 
been  made  to  convert  them  into  Portland  cement  or  similar 
hydraulites.  As  blast-furnace  slags  are  very  hard  and  difficult 
to  grind  they  are  run  direct  from  the  furnace  into  water  and 
so  become  reduced  to  a  coarse  powder  or  granulated. 

Blast  furnace  slags  may  be  divided  into  three  classes  :— 

(a)  Slags  of  such  a  composition  that  when  granulated  and 


SLAGS  17 

ground  they  form  a  kind  of  Portland  cement,  best  termed 
slag  cement,  as  it  contains  less  lime  and  more  alumina  than 
true  Portland  cement. 

(b)  Slags    which,    when    granulated    and    ground,    form    a 
pozzolanic  material,  i.e.,  a  material  which  must  be  mixed  with 
lime  before  it  becomes  hydraulic. 

(c)  Slags  of  an  acid  character  which  must  be  mixed  with 
limestone  and  calcined.     Such  slags  simply  replace  the  clay 
ordinarily  used  in  the  manufacture  of  Portland  cement. 

Although  it  is  conceivable  that  cements  may  be  produced 
from  basic  blast-furnace  slags  with  a  composition  and  properties 
closely  resembling  those  of  true  Portland  cement,  it  is  difficult 
to  do  this  at  a  sufficiently  low  cost  to  make  the  manufacture 
commercially  profitable  in  countries  where  suitable  clays  are 
available.  In  the  United  States  this  method  is  largely  used 
as  a  means  of  getting  rid  of  the  enormous  quantities  of  slag 
produced  in  the  large  ironworks,  the  charges  for  tipping  and 
storing  on  land  being  thereby  avoided.  The  slag  is  very 
cheap  though  the  grinding  is  costly,  and  the  fact  that  it  is 
a  by-product  prevents  its  composition  being  materially  altered 
in  the  furnace.  The  addition  of  more  limestone  to  the  contents 
of  the  furnace  will  usually  result  in  the  production  of  a  mass 
which  is  too  viscous  to  be  run  off,  it  being  seldom  possible  to 
produce  a  fluid  slag  containing  more  than  50  per  cent,  of  lime, 
whilst  a  good  Portland  cement  contains  more  than  60  per  cent, 
of  this  base. 

The  production  of  a  pozzolanic  slag  is  much  easier  than  that 
of  a  Portland  cement  from  the  same  slag,  and  large  quantities 
of  such  are  now  made.  To  obtain  satisfactory  results,  the 
slags  must  have  a  composition  within  somewhat  narrow  limits, 
such  as  the  following  :  silica  30  to  36  per  cent.,  alumina  and 
iron  oxide  12  to  17  per  cent.,  lime  48  to  50  per  cent.,  and 
magnesia  under  3  per  cent.  Like  all  pozzolanas,  such  slags 
only  form  cements  when  ground  with  lime  and  water. 

The  utilisation  of  blast-furnace  slag  in  cement  manufacture 
has  attracted  much  attention  for  a  long  time,  but  the  difficulties 
in  handling  the  material  are  so  great  that  much  remains  to  be 
done.  At  present,  only  basic  slags  can  be  used  for  the  purpose, 
and  the  methods  of  granulation  and  of  reduction  to  powder 

c.  c 


18          THE   RAW  MATERIALS    FOR    CEMENTS 

present  difficulties  almost  as  great  as  do  the  variations  in  the 
composition  of  the  slags. 

(e)  Sundry  Raw  Materials  used  in  the  Manufacture 
of  Cements. 

IRON  ORE. 

Iron  ore  has  been  largely  used  in  the  manufacture  of  a  kind 
of  Portland  cement  used  in  the  Panama  Canal  and  for  various 
marine  works.  The  iron  ore  is  used  in  place  of  clay,  and  the 
process  of  manufacture  is  precisely  the  same  as  that  used  for 
producing  Portland  cement  from  shale  and  limestone.  The 
cement  produced  has  a  composition  closely  resembling  that  of 
Portland  cement  in  which  the  alumina  has  been  replaced  by 
iron  oxide.  W.  Michaelis  has  published  the  following  analysis  of 

IRON-ORE  CEMENT  :— 

Silica 23-26 

Alumina      .....  1-67 

Iron  oxide  .          .          .          .          .  8-20 

Lime 64-84 

Magnesia     .          .          .          .          .  0-66 

Sulphur  trioxide  (S03)  .          .          .  1-08 


99-71 
ALKALI-WASTE. 

For  many  years  the  waste  produced  by  alkali  works  was 
unusable  ;  it  consists  chiefly  of  calcium  carbonate  with  a 
considerable  proportion  of  sulphides.  Now  that  the  sulphur 
can  be  removed,  the  purified  "  waste  " — in  the  form  of  a  fine 
slurry — is  admirably  adapted  for  the  manufacture  of  Portland 
cement.  For  this  purpose  it  is  "  blunged  "  in  a  wash-mill 
with  a  suitable  quantity  of  clay  and  water,  and  the  mixed 
slurry  is  run  off  to  a  dryer.  Alkali-waste  containing  a  large 
percentage  of  calcium  sulphate  is  unsuitable  for  cement 
manufacture. 

The  Le  Blanc  process  waste  is  inferior  to  ammonia  process 
waste  for  the  manufacture  of  Portland  cement. 


ALKALI-WASTE  19 

The  following  is  an  analysis  of  alkali-waste  : — 

Silica 1-98 

Alumina      .          .          .          .          .  1-41 

Iron  oxide  .....  1-38 

Calcium  carbonate         .          .          .  86-27 

Magnesia     .          .                     .          .  2-84 

Sulphuric  acid       .                     .          .  1-26 

Potash  and  soda  .          .          .          .  0-65 

Water,  etc.            ....  4-21 

If  sufficient  care  is  taken  in  freeing  the  waste  from  sulphur 
compounds,  in  adding  the  correct  proportion  of  clay,  and  in 
securing  a  finely-ground  and  well-burned  clinker,  the  .use  of 
alkali-waste  is  highly  satisfactory.  Unfortunately,  the  waste, 
after  purification,  is  often  more  costly  than  if  limestone  is 
used. 


c  2 


CHAPTER  II 

METHODS  OF  CEMENT  MANUFACTURE 

HYDRAULIC  cements  are  manufactured  by  a  variety  of 
processes  in  which  the  underlying  principles  are  practically 
identical  ;  they  consist  of  (a)  the  preparation  of  a  suitable 
and  uniform  material,  (b)  heating  it  to  a  suitable  temperature, 
and  (c)  reducing  the  calcined  product  (clinker)  to  the  state 
of  a  very  fine  powder.  In  some  cases  (as  in  natural  and  rock 
cements)  only  one  material  is  used,  and  a  large  part  of  the 
first  stage  of  manufacture  may  be  omitted  ;  in  others  (as  the 
pozzolanas)  the  material  is  obtained  after  natural  calcination 
by  volcanic  action  and  the  second  stage  may  be  omitted  :  but 
in  spite  of  these  modifications  the  general  principles  apply  in 
all  cases. 

PORTLAND  CEMENT. 

Portland  cements  are  chiefly  made  by  mixing  some  form  of 
limestone,  chalk  or  other  lime-compound  with  some  form  of 
aluminosilicic  acid  (clay),  the  materials  used  being  chosen  for 
their  cheapness,  accessibility  and  general  convenience.  The 
most  important  of  these  materials  have  already  been  described 
(pp.  3,  5,  et  seq.). 

The  development  of  machinery  used  in  the  cement  industry 
has  brought  about  so  great  an  improvement  in  the  preparations 
of  the  raw  materials  that  it  is  no  longer  difficult  to  produce  a 
good  Portland  cement  from  any  suitable  materials.  The 
selection  of  the  best  method  of  working  is,  consequently,  much 
easier  now  than  formerly,  though  the  difficulties  experienced 
in  some  cases  are  made  far  more  serious  in  consequence  of  the 
stress  of  competition.  A  scheme  of  general  applicability  to 
be  employed  by  all  firms  can  never  be  given,  because  in  each 
case  local  conditions  must  receive  proper  consideration. 

According  as  the  raw  materials  are  of  a  soft  and  open,  or 


MANUFACTURE    OF   PORTLAND   CEMENT        21 

hard  and  compact  nature,  two  entirely  different  methods  of 
manufacture  are  employed,  and  are  respectively  known  as  the 
"  wet  "  process  and  the  "  dry  "  process. 

In  the  "  wet  "  process,  which  is  the  older  of  the  two,  the 
materials  must  be  sufficiently  soft  to  form  a  cream  or  slurry 
when  vigorously  stirred  with  water.  Hard,  rocky  materials 
cannot,  therefore,  be  treated  in  this  manner,  and  the  wet 
process  is  largely  confined  to  the  use  of  chalk  with  clays,  marls  or 
muds.  These  are  thrown,  in  suitable  proportions,  into  a  wash- 
mill — a  circular  brickwork  tank  about  fourteen  feet  diameter, 
built  below  the  surface  of  the  ground,  which  is  fitted  with 
rotating  arms  or  harrows  so  arranged  that  they  completely 
sweep  every  part  of  the  mill  and  thoroughly  stir  and  mix  the 
contents.  Sufficient  water  is  added  and  the  harrows  rotated 
by  steam  or  electrical  power,  whereby  the  chalk  and  clay  are 
rapidly  churned  into  a  slurry.  Some  wash-mills  are  arranged 
to  work  continuously,  the  solid  materials  and  water  being 
added  at  regular  intervals  and  the  slurry  flowing  away  in  a 
continuous  stream.  Other  mills  are  charged,  set  in  motion 
so  as  to  convert  the  whole  material  into  a  slurry,  and  then  the 
harrows  are  stopped  and  the  slurry  is  run  off  through  a  grating. 
The  intermittent  system  of  working  is  slower,  but  wastes  less 
material  and  is  considered  to  produce  a  more  uniform  mixture. 

In  either  case  the  flints,  stones  and  other  coarse  impurities 
in  the  raw  materials  sink  to  the  bottom  of  the  mill,  and  are 
either  removed  continuously  by  means  of  a  bucket  elevator, 
or  they  are  removed  from  the  mill  after  the  slurry  has  been 
run  off. 

In  order  that  the  output  of  the  wash-mill  may  be  as  large 
as  possible  it  is  customary  to  break  up  lumps  of  chalk  more 
than  three  inches  diameter  with  small  picks  or  hammers  or 
to  pass  the  chalk  over  a  very  coarse  screen,  the  rejected  portion 
being  crushed  by  rollers  and  again  passed  on  to  the  screen. 
To  keep  the  slurry  passing  out  of  the  mill  free  from  coarse 
material  a  fine  grid  or  gauze  is  placed  in  the  outlet  pipe.  In 
some  works  catch  pits  for  any  coarse  sand  in  the  slurry  are  also 
used,  the  slurry  flowing  through  these  as  soon  as  it  has  left 
the  mill.  Some  firms  also  grind  their  slurry  in  a  tube  mill  in 
order  to  ensure  the  complete  absence  of  coarse  particles.  The 


22    METHODS  OF  CEMENT  MANUFACTURE 

slurry  is  next  run  into  settling  tanks  or  "  backs  "  where  the 
solid  portion  settles  and  the  clear  watei  is  run  off.  As  soon 
as  the  material  in  the  "  backs  "  is  fairly  stiff  and  has  attained 
the  consistency  of  butter,  it  is  taken  to  the  mixer  and  then 
to  the  drying  floors. 

It  is  of  the  greatest  importance  that  the  mixture  should  be 
quite  uniform  in  composition,  as  small  variations  will,  in  some 
cases,  create  serious  differences  in  the  product.  Its  fineness 
and  its  chalk-content  must  therefore  be  tested  several  times 
daily. 

As  the  settling  of  the  material  in  the  "  backs  "  is  a  slow 


FIG.   1. — Johnson's  Kiln. 

process  and  tends  to  cause  a  separation  of  the  various  ingre- 
dients, whilst  most  of  the  water  remaining  in  the  paste  must 
be  all  dried  out  before  the  material  can  be  sent  to  the  kilns, 
many  attempts  have  been  made  to  reduce  the  amount  of  water 
used  whilst  still  obtaining  a  material  so  fine  that  only  4  per 
cent,  will  remain  on  a  sieve  with  180  holes  per  linear  inch. 
The  process  which  has  proved  most  successful  is  based  on  a 
suggestion  of  Goreham,  who  recommended  the  addition  of 
only  about  15  per  cent,  of  water  to  the  materials,  which  are 
therefore  converted  into  a  much  thicker  slurry.  The  wash- 
mill  is  the  same  as  that  previously  described,  but  a  grating 
with  | -inch  openings  is  used  instead  of  the  fine  sieve,  and  the 
slurry  is  ground  between  mill  stones  or  in  a  tube-mill  consisting 


MANUFACTURE   OF   PORTLAND   CEMENT        23 


of  a  cylinder  rotating  about  its  longitudinal  axis  and  containing 
heavy  steel  balls.  To  ensure  the  absence  of  coarse  particles 
of  flint,  etc.,  the  slurry  is  then  sieved  before  being  passed  to 
the  mixing  tanks  and  thence  to  the  rotary  kilns. 

Where  one  material  is  much  harder  than  the  other  it  is 
treated  separ- 
ately, and  the 
slurries  from  each 
mill  are  mixed 
in  suitable  pro- 
portions. This 
arrangement  se- 
cures a  larger 
output  of  a  more 
uniform  character, 
as  if  the  clay  is 
washed  separately 
from  the  chalk 
the  proportions 
of  each  may  be 
more  accurately 
gauged.  The 
grinding  of  the 
mixed  slurry  in 
a  special  mill  has 
only  been  prac- 
tised in  recent 
years.  It  in- 
creases the  cost 
of  manufacture, 
but  gives  so 
superior  a  product 
that  it  should  seldom,  if  ever,  be  omitted. 

The  paste  produced  from  the  slurry  must  be  dried  to  a 
solid  mass  if  stationary  kilns  are  used,  as  the  material  must  be 
sufficiently  firm  to  resist  the  crushing  action  of  the  material 
above  it.  Some  firms  press  the  paste  into  bricks  and  dry 
these,  but  the  usual  practice  has  been  to  spread  the  paste  on 
a  level  floor  heated  by  waste  gases  from  the  kilns  (Fig.  1). 


FIG.  2. — Modern  Shaft  Kiln. 


24    METHODS  OF  CEMENT  MANUFACTURE 


Owing  to  the  shrinkage  it  undergoes  on  drying,  the  material 
cracks  into  pieces  of  a  convenient  size  and  these  are  placed  in 
*  the    kiln.     Two    forms 

of  stationary  kiln  are 
used — the  shaft  kiln 
(Fig.  2),  of  which  there 
are  numerous  patterns, 
differing  from  each 
other  chiefly  in  the 
arrangements  provided 
for  preventing  the  waste 
of  heat.  A  typical 
example  is  Johnson's 
kiln  (Fig.  1),  in  which 
the  slurry  is  dried  in 
the  part  A  by  waste 
heat  from  the  kiln  B. 
Another  is  the  Hoff- 
mann kiln  (Fig.  3),  in 
which  the  material  is 
slaked  in  a  series  of 
chambers,  the  heat 
from  one  being  used 
to  warm  up  the  others 
so  that  it  is  used  to 
the  best  advantage. 
Both  these  classes  of 
kiln  are  now  being  re- 
placed by  rotary  kilns 
(p.  26),  so  that  no  de- 
tailed description  of 
them  is  needed. 

The  dry  process  is 
suitable  for  almost 
every  kind  of  material, 
though  sticky,  highly  plastic  clays  are  troublesome  unless 
previously  heated  to  destroy  their  plasticity.  This  heating 
is  usually  termed  "  drying,"  though  it  does  more  than  merely 
drive  off  the  moisture  in  the  clays.  The  material  is  usually 


FIG.   3. — Hoffmann  Kiln. 


MANUFACTURE   OF   PORTLAND   CEMENT         25 

passed  through  a  preliminary  crusher  which  reduces  it  to 
pieces  of  a  convenient  size  for  "  drying."  The  "  dried  " 
materials  are  then  mixed  in  suitable  proportions  in  a  mixing 
drum  and  passed  into  storage  bins.  From  these  bins  the 
material  passes  to  the  grinding  mills,  where  it  is  ground  so  fine 
that  not  more  than  4  per  cent,  will  remain  on  a  No.  180  sieve, 
though  the  actual  fineness  is  a  matter  for  experiment,  some 
comparatively  coarse  materials  making  an  excellent  cement. 
The  mills  used  for  this  purpose  are  edge  runners,  ball  mills, 
mill  stones,  centrifugal  mills,  or  disintegrators,  the  last  named 
being  only  suitable  for  coarse  grinding.  Various  screening  or 
sifting  devices  are  employed  to  separate  the  fine  material  and 
to  return  the  coarser  product  to  the  mill  to  be  still  further 
reduced. 

The  ground  material,  termed  raw  meal,  is  stored  in  silos  or 
bins,  each  sufficiently  large  to  hold  about  six  hours'  output. 
To  secure  a  uniform  product  some  means  of  mixing  the  material 
is  employed  in  these  silos,  a  species  of  bucket  elevator,  which 
removes  the  material  from  the  bottom  and  returns  it  to  the 
top  of  the  silo,  being  generally  used.  The  material  in  the  silos 
must  be  tested  for  fineness,  and  its  composition  must  be  adjusted 
by  the  addition  of  clay  or  limestone  powder  if  it  does  riot 
correspond  exactly  to  that  required  to  make  good  cement. 

From  the  mixer  the  raw  material  is  passed  to  a  rotary  kiln 
(Fig.  4),  which  consists  of  an  inclined  steel  tube  lined  with 
firebricks  and  cement  clinker,  100  ft.  or  more  in  length  and 
6  ft.  or  more  in  diameter.  The  raw  meal  is  fed  in  at  the  top 
whilst  the  fuel,  in  the  form  of  dust,  is  blown  in  at  the  other 
end  by  a  blast  of  air  more  than  sufficient  for  its  combustion. 
As  the  kiln  revolves  the  material  travels  slowly  down  the 
tube,  becoming  hotter  and  hotter  until  it  reaches  a  state  of 
partial  fusion  or  sintering,  and  is  eventually  discharged  from 
the  lower  end  of  the  kiln  in  a  white  hot  condition.  Variations 
in  the  shape  of  the  kiln  have  been  made  from  time  to  time,  the 
modern  tendency  being  to  use  kilns  which  are  of  larger  diameter 
in  the  zone  of  greatest  heat  than  they  are  at  either  end.  The 
speed  of  rotation  is  quite  slow — about  thirty  revolutions  per 
hour — and  is  regulated  to  suit  the  material  being  heated. 
The  final  temperature  reached  in  the  kiln  is  about  1410°  C. 


26    METHODS  OF  CEMENT  MANUFACTURE 

The  material  discharged  from  the  lower  end  of  the  kiln  is 
termed  clinker,  and  is  received  into  a  cooling  device  which 
usually  consists  of  a  rotating,  inclined  steel  tube  30  to  50  ft. 
long  and  5  ft.  wide,  which  is  partially  lined  with  firebrick  and 
fitted  with  baffles.  As  the  cooler  rotates,  these  baffles  lift 
the  clinker  and  allow  it  to  fall  in  the  form  of  a  cascade,  so  that 
it  is  brought  into  close  contact  with  a  current  of  air  which  is 
sent  through  the  cooler  by  means  of  a  fan.  The  design  ami 


i 


FIG.  4. — Pfeiffer's  Rotary  Kiln. 


construction  of  a  rotary  kiln  and  its  accessory  coal-grinding 
plant  require  a  large  amount  of  skill.  A  number  of  firms  have 
specialised  in  their  manufacture,  have  brought  them  to  a  high 
degree  of  perfection,  and  have  adopted  many  devices  of 
considerable  importance  in  obtaining  a  first-class  product  in 
the  most  economical  manner.  These  details  are  beyond  the 
scope  of  the  present  volume. 

Cement  clinker  from  stationary  kilns  is  in  the  form  of  irregular 
lumps  ;  that  from  rotary  kilns  is  in  grains  rather  larger  than 
peas.  If  correctly  burned  it  is  a  dark  grey  or  blue  grey  sub- 


MANUFACTURE    OF   PORTLAND   CEMENT        27 

stance,  extremely  hard  and  full  of  minute  pores.  Insufficiently 
burned  clinker  is  buff  coloured  ;  it  must  be  separated  and  re- 
burned.  Over-heated  clinker  can  only  be  produced  when  the 
material  has  been  in  contact  with  a  siliceous  kiln  lining,  as 
clinker  alone  is  not  affected  by  the  greatest  attainable  heat 
in  a  cement  kiln.  In  the  presence  of  silica,  on  the  contrary, 
a  more  siliceous  silicate  is  formed  which  is  not  hydraulic,  and 
is  therefore  useless  for  cement. 

The  cooled  clinker  is  usually  passed  into  a  storage  bin,  from 
which  it  is  drawn  as  required  by  the  men  in  charge  of  the 
grinding  plant.  It  is  a  curious  fact,  and  one  which  in  the 
early  days  of  rotary  kilns  caused  great  inconvenience,  that 
merely  to  reduce  rotary  kiln  clinker  to  a  fine  powder  will  not 
produce  a  satisfactory  cement  ;  such  a  powder  sets  immedi- 
ately, and  some  means  must  be  used,  therefore,  to  delay  its 
setting  for  a  convenient  time.  This  is  usually  accomplished 
by  the  addition  of  about  2  per  cent,  of  water,  preferably  in  the 
form  of  steam,  and  1  per  cent,  of  gypsum.  The  grinding 
machines  are  chiefly  tube  or  ball  mills  consisting  of  rotary 
cylinders  containing  steel  balls  which  fall  like  a  cascade  when 
the  mill  is  in  action,  and,  in  falling,  crush  the  material  to 
powder.  This  powder  is  then  passed  into  a  separator  in  which 
it  falls  on  a  rapidly  rotating  plate,  the  coarse  powder  being 
returned  to  the  mill  whilst  the  fine  powder  is  carried  along  by 
a  current  of  air  and  is  eventually  deposited  in  sacks  or  casks 
or  in  a  silo.  Sieves  may  be  used  in  place  of  separators,  but 
are  understood  to  give  less  fine  flour  in  the  cement. 

Many  arguments  have  been  brought  forward  as  to  the 
relative  advantages  of  the  dry  and  wet  processes,  but  the 
opinion  is  still  held  by  many  people  that  the  wet  process  is 
preferable  wherever  it  can  be  applied.  A  careful  investigation 
of  both  processes  will  fail  to  show  any  material  differences  in 
the  final  product,  and  it  may  be  taken  for  granted  that  with 
equal  care  and  skill  either  method  of  working  will  produce 
good  results.  The  choice  of  one  method  should,  therefore, 
depend  on  the  costs  of  manufacture  and,  as  these  will  differ 
in  different  localities,  it  is  usually  necessary  to  employ  an 
impartial  and  independent  expert  to  go  fully  into  the  question. 
Broadly  speaking,  the  wet  process  is  the  cheaper  for  moist, 


28    METHODS  OF  CEMENT  MANUFACTURE 

soft,  raw  materials  such  as  chalk  and  marly  clays  ;  its  chief 
drawback  being  the  high  cost  of  burning  due  to  the  evaporation 
of  the  added  water.  If  the  dry  process  is  used,  the  raw 
materials  contain  only  10  to  15  per  cent,  of  moisture,  and  this 
is  removed  by  the  waste  gases  of  the  kiln  without  any  cost. 
With  the  wet  process,  on  the  contrary,  the  grinding  is  much 
less  expensive  ;  but  this  small  saving  cannot  counterbalance 
the  high  cost  of  fuel  for  drying  out  the  added  water.  Moreover, 
the  saving  in  power  will  be  nullified  if  the  wet  mills  require 
more  repairs  than  the  dry  ones.  A  well-managed  plant  working 
in  the  wet  way  averages  5  Ibs.  of  medium  quality  coal  to  each 
100  Ibs.  of  cement  more  than  when  the  dry  process  is  used,  as 
the  evaporation  of  the  water  in  the  wet  process  requires  as 
much  heat  as  will  clinker  the  dry  raw  material.  This  is 
equivalent  to  almost  20  per  cent,  of  the  coal  used. 

In  the  earlier  days  of  rotary  kilns,  when  they  were  built 
too  short  for  materials  treated  by  the  dry  process,  the  coal 
burned  was  greater  in  the  dry  than  in  the  wet  process,  and  the 
influence  of  the  fuel  ashes  was  so  great  that  many  firms  were 
driven  to  use  the  wet  process.  Recently,  the  dimensions  of 
rotary  kilns  have  increased  enormously,  and  the  modern  kilns 
can  deal  with  dry-process  material  in  a  perfectly  satisfactory- 
manner.  Other  important  reasons  for  using  the  wet  process 
are  the  absence  of  drying  plant  for  the  raw  material  and  absence 
of  dust,  but  neither  of  these  are  of  primary  importance. 

The  idea  that  the  wet  process  effects  a  better  mixing  of  hard 
materials  and  improves  the  quality  of  the  cement  is  no  longer 
tenable,  and  the  belief  that  less  power  is  required  is  equally 
inaccurate.  The  power  required  is  only  less  when  the  output 
of  a  wet  mill  is  compared  with  that  of  a  ball  or  tube  mill  of 
an  old  pattern  ;  the  modern  dry-process  machines  operate 
very  advantageously  and  satisfy  all  requirements  with  regard 
to  uniformity  and  fineness  of  product  and  power  consumption, 
providing  the  materials  are  suitable  ;  wet  materials  being 
best  treated  by  the  wet  process.  For  hard  materials,  such  as 
limestone  and  shale,  the  dry  process  is  preferable.  In  this 
country,  the  softer  materials  are  available  in  such  large 
quantities  that  the  wet  process  is  more  generally  used  ;  in 
America,  on  the  contrary,  the  harder  materials  are  the 


SAND   CEMENT  29 

more  frequently  employed,  and  the  dry  process  is  preferred 
there. 

SAND  CEMENT. 

Sand  Cement  is  a  mixture  of  equal  weights  of  sand  and 
Portland  cement,  the  two  materials  having  been  ground 
together.  Sand  cement  has  a  tensile  strength  almost  as  great 
as  that  of  neat  Portland  cement,  but  its  value  is  greatly 
exaggerated  by  tensile  and  other  tests,  as  will  be  seen  by  using 
sand  cement  mixed  with  three  times  its  weight  of  sand.  It 
will  then  be  found  that  the  strength  of  the  cement-sand 
mixture  is  much  lower  than  that  of  mixtures  of  Portland 
cement  and  sand  in  the  same  proportion.  The  fact  is  that 
tests  of  the  tensile  strength  of  neat  cements  are  almost  meaning- 
less, and  only  mixtures  of  cement  and  sand  in  the  proportion 
of  1  :  3  give  really  satisfactory  results.  Tests  of  neat  cement 
are  no  longer  made  on  the  Continent  (see  p.  139). 

NATURAL  CEMENTS. 

Long  before  Portland  cement  had  been  invented,  many 
so-called  "  natural  cements  "  were  in  use.  These  were  made 
by  heating  certain  naturally  occurring  materials  (p.  13)  and 
grinding  the  calcined  product.  These  "  natural  "  or  "  rock  " 
cements  are  far  less  uniform  in  composition  than  are  Portland 
cements,  because  no  care  is  taken  to  adjust  the  composition 
of  the  raw  materials  used.  They  are  also  inferior  because 
they  are  generally  under-burned — the  temperature  in  the 
furnace  being  insufficient  to  cause  complete  combination  of 
the  clay  and  lime  or  their  equivalents — as  it  is  under  1200°  C. 
instead  of  1400°  C.  or  above. 

It  is  essential  that  the  materials  from  which  they  are  made 
should  contain  clay  and  limestone  (or  the  equivalent  alumino- 
silicic  acid  and  calcium  carbonate)  in  suitable  proportions, 
and  the  value  of  the  cements  produced  depends  largely  on 
the  composition  of  the  raw  rock. 

The  methods  employed  in  the  manufacture  of  natural  and 
rock  cements  resemble  those  used  for  Portland  cement, 
except  that  there  is  only  one  raw  material  instead  of  two,  and 


30    METHODS  OF  CEMENT  MANUFACTURE 

that  no  efforts  are  made  to  test  and  adjust  the  composition 
of  the  material  during  manufacture.  The  raw  rock  or  septaria 
(p.  13)  are  placed  in  the  kiln  without  being  crushed,  stationary 
kilns  (Figs.  2  and  3)  being  employed.  The  calcined  material 
is  then  ground  to  powder  in  a  similar  manner  to  cement, 
though  the  grinding  is  seldom  so  complete. 

The  clinker  drawn  from  the  kiln  must  be  sorted,  the  light- 
coloured  under-burned  pieces  being  separated  from  the  darker 
clinker  and  used  only  for  inferior  work  or  returned  to  the  kiln 
to  be  re-heated.  Some  manufacturers  claim  that  the  clinkered 
material  is  less  satisfactory  than  a  rather  more  lightly  burned 
product.  These  differences  in  behaviour  are  probably  due 
to  differences  in  the  composition  of  the  clinker  :  the  greater 
the  proportion  of  lime  the  higher  must  be  the  temperature 
inside  the  kiln  to  secure  an  adequate  combination.  If,  on 
the  contrary,  the  burning  has  been  properly  executed,  a  cement 
with  a  large  proportion  of  lime  will  be  stronger  than  one 
containing  less  lime.  If  the  proportion  of  lime  (CaO)  in  the 
raw  rock  or  marl  is  between  1-8  and  2-4  times  that  of  the 
silica  and  alumina,  a  good  natural  cement  will  be  obtained  at 
a  temperature  of  about  1150°  C.,  but  if  the  lime  exceeds  four 
times  the  "  silica  +  alumina,"  the  temperature  needed  will  be 
as  high  as  that  required  for  Portland  cement.  The  absence 
of  accurate  knowledge  of  the  composition  of  the  materials 
and  variations  in  the  temperature  of  the  kilns  used,  usually 
results  in  a  considerable  proportion  of  over-  and  under-burned 
material  being  produced,  and  it  is  not  unusual  for  one  quarter 
of  the  contents  of  a  kiln  to  be  rejected. 

Natural  cements  are  usually  much  coarser  than  Portland 
cements,  but  during  the  last  few  years  finer  grinding  has  been 
customary  so  as  to  be  better  able  to  compete  with  Portland 
cement. 

The  great  drawback  to  natural  and  rock  cements  is  their 
unreliability.  At  the  present  time,  their  chief  purpose  appears 
to  be  to  form  a  cheap  rival  to  Portland  cement.  The  superiority 
of  the  latter  is  so  great,  however,  that  manufacturers  are 
finding  it  will  pay  them  better  to  take  more  pains  to  secure  a 
uniform  product  of  a  composition  and  properties  practically 
identical  with  those  of  Portland  cement.  Some  of  them  have, 


MANUFACTURE  OF  NATURAL  CEMENT    31 

therefore,  installed  arrangements  for  testing  and  adjusting  the 
composition  of  the  raw  material  and  of  treating  it  in  the  same 
manner  as  in  making  Portland  cement.  The  process  is  certainly 
more  costly,  but  the  better  prices  obtained  fully  warrant  the 
additional  expense.  With  natural  materials  so  nearly  correct 
in  composition  it  seems  unfortunate  that  firms  should  continue 
to  produce  so  inferior  a  product  as  natural  cement  when  they 
might  so  advantageously  manufacture  Portland  cement.  To 
do  this  it  is  essential  that  the  material  should  be  ground  to 
powder  and  thoroughly  mixed  before  it  enters  the  kiln,  as  the 
direct  calcination  of  relatively  large  lumps  is  one  of  the  chief 
causes  of  irregular  composition.  The  two  chief  constituents 
of  the  material  are  not  in  sufficiently  intimate  contact  to 
produce  a  uniform  product  unless  the  raw  material  has  first 
been  reduced  to  powder. 

Roman  cement  (p.  13)  is  one  of  the  oldest  of  the  natural 
cements,  but  its  name  is  quite  misleading,  as  the  material 
bears  no  resemblance  to  the  mortar  used  by  the  -ancient 
Romans.  It  was  first  made  in  England  in  1796,  the  raw 
material  being  the  sept  aria  (p.  13)  or  clayey  nodules  dredged 
from  the  sea  near  Harwich  and  Sheppey.  These  nodules  are 
calcined  lightly  in  a  shaft  kiln  (Fig.  2)  and  are  then  reduced 
to  a  rather  coarse  powder. 

Whilst  useful  where  Portland  cement  is  not  available,  Roman 
cements  can  only  be  regarded  as  crude  inferior  products  of  a 
similar  type,  their  disadvantages  being  due  to  their  irregularity 
in  composition  and  the  coarseness  of  the  product,  but  their 
low  cost — about  half  that  of  Portland  cement — causes  them 
to  be  largely  used  in  some  localities. 

Belgian  cement — sometimes,  but  erroneously,  sold  as  Belgian 
Portland  cement — is  a  natural  cement  manufactured  in  the 
district  of  Tournai,  where  apparently  inexhaustible  quarries 
of  clayey  limestone  occur.  This  material  (see  analysis  on 
p.  13)  when  calcined,  very  closely  resembles  Portland  cement 
clinker,  and  is  only  distinguished  from  it  with  difficulty. 

Belgian  cement  is  made  with  greater  care  than  most  natural 
cements,  but  it  is,  nevertheless,  very  inferior  to  Portland 
cement  on  account  of  the  lower  temperature  at  which  it  is 
burned  and  the  coarseness  of  the  final  product.  Good 


32         METHODS   OF   CEMENT   MANUFACTURE 

genuine  Portland  cement  is  undoubtedly  produced  in  Belgium, 
but  what  is  known  as  "  Belgian  cement  "  is  quite  different, 
and  is  notoriously  inferior,  very  unreliable,  and  often  even 
dangerous.  The  prudent  professional  man  should  never  allow 
its  use  on  works  under  his  control.  This  "  Belgian  cement  " 
is  no  better  than  a  hydraulic  lime,  and  is  made  in  the  same 
manner.  There  is  no  careful  mixing  of  two  separate  raw 
materials,  with  all  the  refinements  of  scientific  control  at  every 
stage  of  the  process  and  at  every  hour  of  the  day  in  order  to 
remedy  the  ever-present  variations  in  the  chemical  composition 
of  the  materials  employed,  and  to  secure  a  uniform  chemical 
composition  of  the  resulting  product,  by  which  alone  a  uniform 
quality  can  be  obtained.  The  Belgian  rock,  which  varies 
greatly  in  its  chemical  composition  and  is  usually  deficient  in 
lime,  is  taken  just  as  it  comes  from  the  quarry,  burned  and 
ground  exactly  as  if  it  were  to  be  sold  as  "  hydraulic  lime  "  ; 
indeed,  much  of  it  is  sold  under  that  name  in  its  own  country. 
But  some  wily  vendors,  keenly  alive  to  the  value  of  a  name, 
know  by  experience  that  many  persons  who  would  not  touch 
it  under  a  true  description  will  buy  readily  enough  if  it  be  sold 
as  "  Portland  cement,"  or  "  best  Portland  cement,"  and  they 
therefore  pack  it  in  casks  exactly  like  that  used  for  genuine 
Portland  cement,  are  ever  ready  to  attach  any  label  preferred 
by  the  purchaser,  and  more  often  than  not  print  the  label 
in  the  English  language,  call  their  firm  by  an  English  name,  and 
do  everything  they  can  to  make  the  guileless  consumer  imagine 
he  is  buying  genuine  Portland  cement  produced  by  English 
makers,  whose  reputation  throughout  the  world  still  stands 
as  a  guarantee  of  high  and  reliable  quality.  In  short,  the 
sooner  the  use  of  the  word  "  Portland  "  in  connection  with  such 
Belgian  cement  is  stopped,  the  better. 

QUICK  AND  HYDRAULIC  LIMES. 

Hydraulic  limes  are  prepared  by  burning  limestones  con- 
taining clay  (p.  4)  in  a  manner  precisely  similar  to  that  used 
in  the  manufacture  of  quick-lime.  The  material  is  placed 
in  a  shaft  kiln  with  alternate  layers  of  coal,  and  the  burned 
lime  is  drawn  out  at  the  bottom  of  the  kiln.  In  order  to  avoid 


MANUFACTURE   OF  HYDRAULIC   LIME          33 

the  admixture  of  ashes  from  the  fuel  with  the  burned  lime, 
gas  may  be  used  instead  of  coal,  and  where  the  output  is  large 
various  methods  are  used  for  keeping  the  kilns  continuously 
at  work.  With  smaller  kilns,  the  usual  custom  is  to  fill,  burn 
and  empty  them,  treating  each  kiln  separately  from  the  rest. 
This  arrangement  has  the  advantage  of  keeping  the  lime  in 
larger  lumps  than  when  it  has  to  travel  down  a  tall  shaft  as 
is  the  case  when  the  kilns  are  worked  continuously. 
Horizontal  draught  kilns  of  the  Hoffmann  type  (Fig.  3)  are 
also  used  for  this  purpose.  It  is  important  to  avoid  over- 
heating, whereby  the  lime  becomes  partially  vitrified  or 
sintered.  In  burning  pure  or  fat  lime  this  over-heating  does 
not  readily  occur,  but  the  proportion  of  clay  and  free  silica 
in  hydraulic  limes  renders  special  precautions  necessary  or 
the  lime  will  be  dead-burnt. 

The  difficulty  in  burning  hydraulic  limes  is  increased  by  the 
fact  that  they  require  a  higher  temperature  of  calcination  than 
does  common  (fat)  lime. 

After  burning,  the  material  is  carefully  slaked  by  the  addition 
of  a  suitable  quantity  of  water,  and  the  fine  powder  thereby 
produced  is  then  ready  for  sale.  Properly-made  hydraulic 
lime,  therefore,  needs  no  grinding  and  can  thus  be  produced 
more  cheaply  than  Portland  cement.  Ordinarily,  however, 
lumps  of  material  remain  after  slaking  and  must  be  separated 
by  screening  (see  Grappier  cement,  p.  34). 

The  chief  difference  between  hydraulic  lime  and  quick-lime 
is  the  clay  in  the  former  which  prevents  the  hydraulic  lime 
from  slaking  readily,  but  enables  it  to  set  when  immersed 
in  water. 

The  raw  materials  used  for  hydraulic  limes  have  been 
described  on  pp.  4,  et  seq. 

Hydraulic  limes  consist  essentially  of  mixtures  of  "  Portland 
cement  "  with  considerable  quantities  of  quick-lime,  the 
proportions  of  each  being  dependent  on  the  amount  of  clay 
in  the  original  limestone  and  on  the  temperature  attained 
during  the  burning.  Some  hydraulic  limes  made  from  blue 
Lias  limestone  have  a  composition  corresponding  almost 
exactly  with  that  of  Portland  cement,  but  others  correspond 
more  nearly  with  a  mixture  of  70  per  cent,  of  Portland  cement 

c.  D 


34    METHODS  OF  CEMENT  MANUFACTURE 

and  30  per  cent,  of  free  lime.  There  is,  at  present,  no  limit 
of  composition  whereby  hydraulic  limes  can  be  distinguished 
from  other  cements,  and  they  are  best  defined  as  made  from 
argillaceous  limestones  and  as  containing  sufficient  cementitious 
material  to  give  hydraulic  properties  to  the  product  and 
sufficient  free  lime  to  enable  the  material  to  slake  on  the 
addition  of  water.  The  advantage  of  the  free  lime  present  is 
that  the  material  can  be  reduced  to  powder  simply  by  the 
addition  of  water  (slaking),  and  so  does  not  need  to  be  ground 
as  does  Portland  cement  clinker.  It  is,  however,  important 
that  no  more  free  lime  should  be  present  than  is  essential  for 
this  slaking,  as  an  excess  of  lime  merely  weakens  the  cementing 
value  of  the  material.  Finely-ground  grappiers  (below)  are 
usually  added  to  increase  the  hydraulic  properties  of  the 
lime. 

One  of  the  most  famous  hydraulic  limes  is  exported  from 
Teil,  in  the  south  of  France,  and  is  considered  to  be  specially 
suitable  for  sea  walls  and  marine  work.  The  following  is  an 
analysis  of  Teil  hydraulic  lime  before  slaking  : — 

Silica  .  .  .  .  .  26-69 

Alumina  .  .  .  T  .  4-24 

Iron  oxide  .  .  .  .  .  trace 

Lime  .  .  .  /  .  68-55 

Magnesia  .  ..  .  .  .  0-52 

Most  English  hydraulic  limes  are  only  moderately  hydraulitic 
and  are  much  feebler  than  some  of  the  French  and  German 
products,  owing  to  their  much  larger  proportion  of  free  lime. 
They  are  improved  by  the  addition  of  5  per  cent,  of  finely- 
ground  plaster  of  Paris,  and  the  product  is  then  known  as 
Scott's  cement  or  selenitic  cement. 

Grappier  cement  consists  of  the  ground  lumps  or  nodules 
which  remain  when  hydraulic  limes  are  screened.  It  is  a  true 
cement,  though  of  a  composition  somewhat  different  from  that 
of  Portland  cement,  being  usually  rather  low  in  alumina  and 
lime.  Grappier  lumps  consist  chiefly  of  the  hydraulite  and  of 
unburned  limestone  ;  if  the  latter  is  present  in  a  large  propor- 
tion the  cement  will  be  useless.  The  following  is  an  analysis 
of  a,  typical  grappier  cement  :— 


POZZOLANAS  35 

Silica            .....  26-5 

Alumina      .....  2-5 

Iron  oxide  .         %,,         .          .          .  1*5 

Lime  .          ...          .          .  63-0 

Magnesia     .....  1-0 

Sulphur  trioxide  (S03)  ,  0-5 
Carbon  dioxide  (CO.,)     . 
Water 
(See  also  p.  52). 

POZZOLANAS. 

Pozzolanas  are  not  true  cements,  but  only  become  so  when 
mixed  with  lime  and  water.  The  raw  materials  composing 
pozzolanas  are,  essentially,  clays  which  have  been  heated  to 
redness  either  by  natural  forces,  such  as  volcanic  action,  or 
artificially  in  kilns.  Pozzolanas  are,  as  regards  their  origin, 
of  three  classes  : — 

(a)  The  direct  products  of  volcanic  action,  usually  found  on 
the  slopes  of  volcanoes,  such  as  pozzolana  proper,  santorin, 
tosca,  tetin  and  trass  (p.  15).     These  pozzolanas  bear  a  close 
resemblance  to  ashes  and  slags. 

(b)  Products  of  the  decomposition  of  certain  igneous  rocks. 
These  are  but  feeble  hydraulites. 

(c)  Artificial  pozzolanas  obtained  by  crushing  lightly-burned 
clay,  ballast,  tiles,  bricks,  etc.     Care  should  be  taken  to  avoid 
clays  which  have  been  heated  to  partial  vitrification.     Some 
blast-furnace  slags,  when  ground,  are  also  pozzolanic. 

SLAG  CEMENTS. 

Certain  basic  blast-furnace  slags,  when  granulated  by  sudden 
cooling  with  water  and  then  ground  to  a  fine  powder,  form 
valuable  cements.  The  sulphur  present  in  the  slags  is  largely 
removed  in  the  form  of  sulphuretted  hydrogen  gas  during  the 
granulation.  Small  percentages  of  various  other  elements 
are  also  present,  but  do  not  appreciably  affect  the  hydraulicity 
of  the  cement. 

The  use  of  slag  as  a  raw  material  for  cements  which,  in  many 
ways,  resemble  Portland  cements,  has  already  been  discussed 

D2 


36    METHODS  OF  CEMENT  MANUFACTURE 

(p.  16),  and  some  of  the  chief  differences  have  been  pointed 
out.  All  slags  are  not  suitable  for  cement  manufacture,  and 
it  is  convenient  to  divide  those  made  from  blast-furnace  slag 
into  three  classes  : — 

(1)  Pozzolanic  slag  cements  which  consist  of  ground  slag  of 
a  pozzolanic  nature,  to  which  sufficient  slaked  lime  is  added 
during  the  grinding  of  the  material  to  bring  the  total  content 
of  calcium  oxide  up  to  63  to  66  per  cent.     Such  a  cement  is 
but  little  better  than  a  hydraulic  lime. 

(2)  Plaster-slag  cements  made  by  mixing  rapidly  cooled  and 
granulated  slags  of  innate  hydraulicity  with  a  considerable 
proportion  of  plaster  of  Paris.     Such  cements  contain  little 
free  lime,  and  are  peculiarly  resistant  to  acids  and  magnesium 
salts.     There  appears  to  be  a  future  for  slags  of  this  neutral 
type — preferably    with  a   neutral,  sulphur-free  substitute  for 
the    plaster — for    maritime  work.     (See  Asch's  suggestion  in 
a  later  section  dealing  with  the  effect  of  sea  water  on  cement.) 

(3)  Iron  Portland  cement ,  which  is  used  in  large  quantities 
in  Germany  and  in  the  Far  East.     The  title  is  far  from  being 
a  satisfactory  one,  as  the  material  is  not  Portland  cement  at 
all,  but  is  produced  from  basic  blast-furnace  slag.     Basic  slag 
is  mixed  with  a  suitable  proportion  of  limestone,  the  material 
being  then  ground  and  burned  in  the  usual  manner.     The 
clinker  is  mixed  with  nearly  half  its  weight  of  granulated  slag 
and  then  ground  to  a  fine  powder. 

Iron  Portland  cement  differs  from  true  Portland  cement  in 
the  materials  from  which  it  is  made  and  in  some  of  the  properties 
it  possesses,  though  in  many  respects  the  two  materials  closely 
resemble  each  other. 

The  composition  of  Iron  Portland  cement  varies  considerably 
in  different  localities  ;  a  fair  average  is  : — 

per  cent. 

Silica 20  to  25 

Alumina  and  iron  oxide       .          .  9  ,,  15 

Lime        .          .          .          .  54  ,,  60 

Magnesia           ....  0-6  ,,  5-0 

Sulphur  tri-oxide  (SOS)        .          .  0-8  „  2-7 

Alkalies   .  0  „  2 


MANUFACTURE   OF   SLAG   CEMENT  37 

The  term  "  iron-Portland  cement  "  is  also  applied  to  blast- 
furnace slag  which  has  been  mixed  with  twice  its  weight,  or 
more,  of  true  Portland  cement.  It  then  resembles  "  sand 
cement  "  (p.  29),  but  is  rather  stronger.  It  is,  of  course, 
weaker  than  pure  cement. 

The  Iron  Portland  cement  above  mentioned  must  not  be 
confused  with  the  Iron  Ore  cement  mentioned  on  p.  18. 

Slag  cements  are  essentially  mixtures  of  slaked  lime  and 
slag,  and  are  therefore  of  the  nature  of  pozzolanic  cements, 
the  slags  being  regarded  as  a  kind  of  artificial  pozzolana. 
Some  firms  produce  a  kind  of  Portland  cement  in  which  slag 
takes  the  place  of  the  clay  and  part  of  the  limestone  or  chalk, 
and  in  this  way  less  fuel  is  used  than  in  the  manufacture  of 
ordinary  Portland  cement,  the  temperature  at  which  the 
slag-lime  mixture  clinkers  being  lower  than  that  needed  in  a 
Portland  cement  kiln.  The  disadvantage  of  slag  used  in  this 
manner  is  that  the  granulation  of  the  slag  introduces  a  large 
amount  (20  to  40  per  cent.)  of  water  which  must  be  driven  off 
by  heat  before  the  mixture  can  be  calcined. 

The  slag  is  run  from  the  furnace  in  the  form  of  a  white  hot 
molten  stream,  which  is  reduced  to  porous  grains  of  slag-sand 
by  contact  with  water.  Various  methods  of  effecting  this 
granulation  are  in  use,  one  of  the  most  satisfactory  consisting 
in  allowing  the  stream  of  slag  to  flow  into  a  trough  containing 
a  rapid  stream  of  water.  Sometimes  a  jet  of  steam  or  water 
is  allowed  to  play  on  the  slag  before  it  reaches  the  trough. 

Granulation  plays  an  important  part  in  the  manufacture  of 
slag  cement,  for  it  not  only  reduces  the  material  to  a  coarse 
powder  in  a  simple  and  cheap  manner,  but  the  water  appears 
to  have  a  chemical  action,  as  the  granulated  slag  has  much 
stronger  hydraulic  properties  than  slag  which  has  been  more 
slowly  cooled.  In  fact,  the  hydraulic  properties  of  slowly- 
cooled  slag  are  almost  negligible. 

The  porous  granulated  slag  cannot  be  ground  direct  on 
account  of  the  water  it  contains.  It  must,  therefore,  be  dried 
at  a  temperature  below  a  red  heat  in  an  automatic  dryer. 
Such  a  dryer  consists  preferably  of  two  concentric  pipes  slightly 
inclined  from  the  horizontal.  The  material  is  passed  through 
the  annular  space  between  the  pipes  and  hot  air  is  passed  over 


38    METHODS  OF  CEMENT  MANUFACTURE 

it,  so  that  the  wet  material  enters  the  machine  at  one  end  and 
emerges  sufficiently  dry  at  the  other. 

The  dried  slag  is  next  mixed  with  a  suitable  proportion  of 
slaked  lime,  usually  about  35  parts  of  lime  to  100  of  slag 
being  used.  The  mixture  is  then  reduced  to  a  fine  powder  in 
a  tube-mill.  The  mixing  is  usually  effected  automatically, 
and  without  requiring  any  special  attention,  by  charging  the 
mills  used  for  fine  grinding  with  the  various  raw  materials  in 
the  desired  proportions.  The  grinding  machinery  is  the  same 
as  is  used  for  Portland  cement  (p.  25). 

Slag-cements  set  so  slowly  that  they  must  usually  be 
accelerated  by  the  addition  of  calcined  silica,  highly  aluminous 
slag,  or  caustic  alkali. 

Slag-cements  are  much  used  as  adulterants  of,  or  substitutes 
for,  Portland  cement,  but  differ  from  the  latter  in  the  lower 
proportion  of  lime  and  alumina  they  contain  and  in  the  pro- 
portion of  calcium  sulphide  present.  The  chief  distinction  to 
be  found  is  in  the  4  to  8  per  cent,  loss  which  occurs  on  the 
ignition  of  slag  cements,  due  to  the  water  they  contain. 


A         ROTARY    KILN B 


OAL  HOPPCH 


^H    Rlf-LED    COOLINO  CYLIN^R? 


Diagram  of  Modern  Rotary  Kiln  and  Cooling  Cylinders. 


CHAPTER  III 

THE    CHEMICAL   AND    PHYSICAL   CHANGES   IN    CEMENTS 

THE  chemical  and  physical  changes  which  occur  in  the 
manufacture  and  use  of  cements  are  both  complex  and  difficult 
to  investigate.  They  may  be  more  easily  studied  by  separating 
them  into  groups  :  (a)  the  changes  which  occur  in  the  manu- 
facture of  cement  from  the  raw  materials,  and  (6)  the  changes 
occurring  when  the  cement  is  used. 

Changes  in  Manufacture.  —  From  what  has  been  stated  in 
previous  chapters,  it  will  be  understood  that  the  various 
cements  described  are  formed  essentially  from  an  acid  substance 
corresponding  to  aluminosilicic  acid  in  combination  with  a 
basic  substance.  The  acid  and  base  occur  quite  separately 
in  the  form  of  clay,  pozzolanas,  etc.,  and  chalk  or  limestone 
respectively,  or  as  a  mixture  (marl,  argillaceous  limestone,  and 
the  raw  materials  used  for  natural  cements).  Combination 
only  occurs  when  the  mixture  is  heated  to  a  suitable  tempera- 
ture, and  it  is  during  this  heating  that  the  most  important 
"  changes  during  manufacture  "  occur. 

No  perfectly  pure  clay  occurs  in  nature,  and  this  still  further 
complicates  the  problem.  Most  of  the  clays  used  in  the 
manufacture  of  cement  appear  to  consist  of  a  mixture  of  what 
may  be  termed  clay  substance  (aluminosilicic  acid)  together 
with  free  silica  and  other  (non-plastic)  minerals.  As  naturally- 
produced  mixtures  must  vary  greatly  in  composition,  no  single 
chemical  formula  can  be  assigned  to  natural  clays.  The  best 
way  is  to  consider  the  essential  constituent  of  each  of  such 
clays  as  the  chief  factor  in  the  acid  portion  of  the  cement,  the 
free  silica  and  other  minerals  being  considered  S3parately.  In 
other  words  it  is  necessary  first  to  consider  the  action  of  the 
heat  on  each  of  the  constituents  of  the  raw  materials  apart 
from  each  other,  and  then  to  study  what  reactions  occur  when 
the  substances  produced  by  the  action  of  heat  are  brought  into 


40   CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

intimate  contact  with  each  other.  In  the  case  of  cement- 
burning,  both  these  sets  of  changes  occur  at  the  same  time  in 
different  portions  of  the  material,  and  their  separate  considera- 
tion is  only  a  convenient  means  of  explaining  what  actually 
occurs.  It  is  also  convenient  to  distinguish  as  far  as  possible 
between  the  chemical  changes  which  take  place  on  heating  the 
raw  materials  and  the  purely  physical  changes  which  occur 
simultaneously. 

The  Chemical  Action  of  Heat  on  Clay  Substance. 

The  first  effect  of  heat  on  clay  is  to  drive  out  any  moisture 
it  may  contain,  either  in  the  free  state  or  in  the  form  of  water 
absorbed  by  any  colloidal  matter  present.  Some  clays  contain 
as  much  as  30  per  cent,  of  such  water  without  appearing  to  be 
really  wet.  The  changes  which  occur  during  this  drying  are 
purely  physical  so  long  as  the  temperature  is  not  appreciably 
above  100°  C.,  and  will  be  described  later. 

If  the  temperature  is  raised,  chemical  decomposition  occurs 
with  appreciable  rapidity,  and  at  500°  to  800°  C.  a  quick 
evolution  of  water  occurs.  The  precise  nature  of  the  decom- 
position, of  which  this  is  a  sign,  is  not  clearly  understood. 
Rebuffat,  Mellor  and  Holdcroft,  and  others  maintain  that  the 
clay  molecule  is  completely  decomposed  into  a  mixture  of  free 
oxides — silica,  alumina  and  water  ;  but  other  chemists  maintain 
that  an  anhydride  or  similar  compound  of  alumina  and  silica 
is  formed.  Thus,  W.  and  D.  Asch  believe  that  clays  are 
aluminosilicic  acids,  or  the  corresponding  hydrates,  in  which 
the  various  atoms  are  arranged  in  a  series  of  hexagons  and 
pentagons  resembling  the  well-known  "  benzene  ring "  of 
organic  chemistry  and  they  represent  a  molecule  of  the  purest 
clay  obtainable  (refined  china  clay)  as 
(OH)2OH  OH 


CHEMICAL   CONSTITUTION   OF   CEMENT          4i 


Other    and     less   pure    clays    have     corresponding    formulae, 
depending  on  their  composition  (see  p.  5). 

A  complete  statement  of  Asch's  theory  is  too  lengthy  and 
complex  to  be  included  in  the  present  volume,1  but  its  general 
correctness  has  been  confirmed  in  various  ways.  It  is  of  great 
value  in  explaining  the  changes  which  occur  whilst  the  materials 
are  in  the  kiln.  As,  according  to  Asch's  theory,  clays  are 
merely  crude  aluminosilicic  acids,  the  reactions  which  occur 
on  heating  a  clay  with  a  suitable  proportion  of  lime  consists 
chiefly  in  displacing  some  of  the  hydrogen  atoms  in  the  clay 
by  lime  or  magnesia,  just  as  the  corresponding  hydrogen  in  a 
complex  organic  acid  is  replaced  by  a  base  when  the  acid  is 
neutralised.  Any  study  of  the  reactions  is  complicated  by  the 
fact  that  the  clays  and  bases  (lime  or  chalk)  are  far  from  pure. 
The  precise  nature  of  the  compound  formed  must  depend 
largely  on  the  extent  to  which  the  reaction  is  allowed  to  occur 
as  well  as  on  the  particular  aluminosilicic  acid  originally 
present.  Fortunately  for  the  manufacturer  all  aluminosilicic 
acids  (clays)  act  as  though  they  are  partially  decomposed  at 
high  temperatures,  and  form  one  of  three  types  of  stable 
compounds,  viz.  : — 


1. 


12Si0    =8i    All  All  Si 


Si    Al   Al 
~~\/\/ 


8i 


3. 


l2Si02  =  |  Si 
\/ 


Al    Al 


Thus,  a  clay  of  the  type  6Al20Bl2Si02l2H20  will,  on  heating, 
lose  water  and  silica,  or  water,  silica  and  alumina,  according 
to  the  temperature  and  duration  of  heating,  and  will  form  one 
of  the  types  above  mentioned. 

1  Readers  with. a  sufficient  knowledge  of  chemistry  should   consult  "The  Sili- 
cates," by  W.  &  D.  Asch,  published  by  Constable  &  Co.,  Ltd.,  London, 


42    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

The  effect  of  this  particular  decomposition  is  to  convert  all 
clays  into  one  or  more  of  the  above  types  of  compounds,  and, 
consequently,  enables  commercially  useful  Portland  cements 
to  be  made  from  almost  any  material  containing  a  sufficient 
proportion  of  clay. 

Whatever  substances  are  produced  by  the  direct  action  of 
heat,  it  is  generally  recognised  that  their  nearest  natural 
equivalents  are  the  trasses  and  pozzolanas  already  described 
(p.  14)  and  corresponding  (e.g.)  to  3#,0  .  ±RO  .  3AW3 .  ISSiO.,. 

That  a  decomposition  of  the  clay  molecule,  and  not  a  mere 
evolution  of  water  of  crystallisation  occurs  when  clay  is  heated 
cannot  be  doubted,  and  Sokoloff  has  shown  that  the  production 
of  a  pozzolanic  material  of  maximum  hydraulicity  occurs 
precisely  at  the  point  where  a  clay  loses  the  whole  of  its  con- 
stitutional water.  This  production  of  pozzolanas  by  heating 
clays  to  a  temperature  of  about  500°  C.  appears  to  be  charac- 
teristic of  most,  if  not  all,  clays,  and  is  strong  evidence  of  the 
existence  of  a  definite  type  of  essential  constituent  or  "  clay 
substance  "  in  all  clays,  even  though  its  composition  and 
the  arrangement  of  the  atoms  within  the  molecule  may  vary 
considerably. 

The  action  of  more  intense  heat  on  clays  is  extremely  difficult 
to  study.  The  only  definitely  crystalline  substance  which  is 
produced  by  heating  pure  clay  to  the  highest  available  tem- 
perature and  allowing  it  to  cool  is  sillimanite  (SiAl205).  This 
substance  appears  to'  be  formed  at  1200°  C.,  but  whether  it  is 
a  decomposition  product  of  a  more  complex  alumino-silica 
compound  or  whether  it  is  produced  by  the  recombination  of 
silica  and  alumina  set  free  at  a  temperature  of  800°  C.  has  not 
yet  been  determined,  though  the  latter  appears  to  be  the  more 
probable  explanation.  Indeed,  Mellor  and  Holdcroft  regard 
the  formation  of  sillimanite  at  1200°  C.  as  strong  evidence  of 
the  complete  dissociation  of  clay  into  free  silica  and  alumina 
at  a  lower  temperature.  There  is,  however,  remarkably  little 
evidence  as  to  the  true  composition  of  this  crystalline  substance. 
It  resembles  sillimanite  in  several  respects,  but  is  not  im- 
probably one  of  the  three  compounds  whose  structural  formulae 
are  given  on  p.  41.  The  quantities  of  crystals  available  for 
analysis  are  far  too  small  for  a  clear  distinction  to  be  made 


ACTION   OF   HEAT   ON  SILICA  43 

between  SiAl.205  and  the  three  types  of  compounds  mentioned, 
though  the  formation  of  the  latter  is,  theoretically,  far  more 
probable  than  the  complete  dissociation  of  the  clay  into  free 
silica  and  alumina,  and  its  recombination  into  sillimanite. 

In  the  present  volume,  the  product  of  the  direct  action  of 
heat  on  clay  alone  is  termed  calcined  clay. 

The  Chemical  Action  of  Heat  on  Free  Silica. 

The  .chemical  action  of  heat  on  free  silica  is  inappreciable  at 
temperatures  below  800°  C.,  but  above  this  temperature  a 
considerable  increase  in  the  volume  of  the  silica  occurs  and 
tridymite,  which  is  apparently  a  polymerised  form  of  silica, 
i.e.,  xSi0.2,  is  produced.  (For  the  action  of  heat  on  a  mixture 
of  free  silica  and  lime,  see  p.  66). 

The  proportion  of  free  silica  in  a  well-made  Portland  cement 
is  extremely  small  ;  together  with  all  the  other  insoluble 
matter  it  should  not  exceed  3  per  cent.  As  it  is  quite  inert 
when  the  cement  is  in  use  the  presence  of  so  small  a  proportion 
is  quite  devoid  of  importance. 

In  the  raw  materials  used  for  the  manufacture  of  cements  a 
notable  proportion  of  colloidal  silica  (Si02xH20)  may  be  present, 
but  is  converted  into  ordinary,  amorphous  silica  on  heating. 
At  one  time  much  stress  was  laid  on  the  distinction  between 
colloidal  (soluble)  and  amorphous  (insoluble)  silica  owing  to 
the  greater  reactivity  of  the  former,  but  this  distinction  is 
valueless  in  the  case  of  all  cements  in  the  manufacture  of  which 
a  high  temperature  has  been  employed,  providing  that  the 
material  is  ground  sufficiently  fine. 

The  effect  of  Heat  on  Free  Alumina. 

The  effect  of  heat  on  free  alumina  is  to  make  it  insoluble  in 
acids  and  very  resistant  to  the  action  of  alkalies.  It  therefore 
becomes  inert.  It  is  doubtful  whether  free  alumina  ever 
exists  really  in  cements  to  any  appreciable  extent.  When 
free  alumina  is  added  in  small  quantities  (4  per  cent.)  to  some 
samples  of  commercial  Portland  cement  it  causes  expansion 
on  gauging ;  on  other  samples  it  appears  to  have  no  action  of 


44   CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

this  kind  even  when  briquettes  made  of  the  mixture  are  kept 
in  boiling  water  for  several  hours. 

Other  minerals  in  clay  will  undergo  various  physical  and 
chemical  changes  when  exposed  to  the  action  of  heat  at 
temperatures  reached  in  the  manufacture  of  cement.  Some 
of  the  complex  aluminosilicates  appear  to  decompose  with  the 
formation  of  simpler  silicates ;  others  lose  water,  and  must, 
therefore,  be  regarded  as  undergoing  some  decomposition, 
though  the  nature  of  this  is  far  from  being  well  understood. 
As  the  temperatures  reached — particularly  in  rotary  kilns — 
are  very  high  (about  1400°  C.),  it  is  not  unreasonable  to  suppose 
that  any  calcium  aluminosilicates  present  in  a  clay  produce 
substances  which  bear  a  close  resemblance  to  the  essential 
constituent  of  cements,  and  that  the  sodium,  potassium  and 
magnesium  aluminosilicates  produce  analogous  substances 
which  may,  however,  be  devoid  of  hydraulic  properties. 

As  the  proportion  of  "  other  minerals  "  in  clays  and  similar 
substances  used  for  cement  making  is  seldom  large,  the  direct 
chemical  action  of  heat  upon  them  may  be  neglected  ;  the 
total  effects  of  their  presence  is  noted  on  a  later  page.  The 
chemical  changes  induced  by  the  reaction  of  these  substances 
on  one  another  are  described  on  p.  72. 

The  Chemical  Action  of  Heat  on  Limestone. 

The  chemical  action  of  heat  on  limestone  is  comparatively 
easy  to  understand.  Chalk  and  limestone  are  essentially 
composed  of  calcium  carbonate  which,  on  heating  to  700°  C. 
or  above,  dissociates  into  free  lime  (CaO)  and  carbon  dioxide 
(C02)  ;  the  latter,  being  a  gas,  escapes  and  leaves  the  free  lime 
behind.  The  extent  to  which  this  decomposition  occurs 
depends  on  the  pressure  of  the  carbon  dioxide  produced  ;  if 
this  gas  is  allowed  to  escape  the  whole  of  the  carbonate  will 
be  converted  into  oxide,  but  if  some  of  the  carbon  dioxide 
remains  in  the  kiln  or  other  appliance  in  which  the  heating 
occurs,  it  will,  in  time,  produce  such  a  pressure  that  no  further 
decomposition  of  the  carbonate  will  take  place.  This  is 
commonly  expressed  by  the  following  equation  :— 

CaC03  I >  CaO  +  CO2, 

which  indicates  the  reversibility  of  the  reaction. 


ACTION   OF   HEAT   ON  LIMESTONE  45 

Lime  which  has  been  heated  to  above  1000°  C.  loses  part  of 
its  power  to  slake  when  water  is  added  to  it,  but  this  is  believed 
to  be  due  to  a  reduction  in  the  surface  area  of  the  material 
rather  than  to  any  molecular  rearrangement. 

Free  lime  does  not  occur  in  properly  made  Portland  cement  ; 
many  statements  to  the  contrary  are  based  upon  erroneous 
conclusions  drawn  from  experimental  observations.  This  is 
due  to  the  ease  with  which  Portland  cement  is  hydrolysed  when 
treated  with  water,  whereby  lime,  originally  in  combination, 
is  set  free.  The  presence  of  free  lime  in  cement  would  be  very 
disadvantageous,  as  when  the  cement  is  in  use  the  lime  hydrates, 
expands,  and  may  easily  cause  dangerous  cracks  in  the  struc- 
ture. Comparative  tests  on  good  Portland  cements,  with  and 
without  the  addition  of  6  per  cent,  of  lime,  showed  that  the 
whole  of  the  expansion  of  the  lime  occurred  when  the  briquettes 
were  exposed  for  twenty-four  hours  to  a  moist  atmosphere. 
The  same  results  are  produced  in  an  insufficiently-burned 
cement,  i.e.,  one  in  which  all  the  lime  and  clay  have  not  entered 
into  combination,  so  that  free  lime  and  free  "  calcined  clay  " 
are  present. 

It  is  because  of  the  danger  caused  by  the  presence  of  free 
lime  that  a  limit  to  the  proportion  of  lime  is  usually  imposed. 
The  best  means  of  controlling  its  presence  is  the  "  expansion 
test  "  described  in  a  later  chapter.  If  a  cement  can  pass  this 
test  the  proportion  of  free  lime  in  it  will  be  insignificant. 

If  a  cement  mixture  containing  an  excess  of  chalk  or  lime- 
stone is  heated  so  strongly  that  the  excess  of  lime  produced  is 
converted  into  the  slow-slaking,  dense  modification,  or  into 
the  cubic  crystalline  form  produced  by  prolonged  exposure  at 
a  temperature  exceeding  1400°  C.,  the  cement  produced  will 
crack  when  it  is  gauged  and  placed  in  water.  The  lime  then 
slakes  so  slowly  that  the  cement  sets  before  the  slaking  is 
complete,  and  the  increased  volume  of  the  slaked  lime  brings 
about  the  cracking,  or  even  complete  disintegration,  of  the 
mass.  Very  small  percentages  of  this  crystalline  lime  will 
render  a  cement  unsound,  but  as  its  presence  is  exceedingly 
difficult  to  detect,  it  is  customary  to  omit  all  search  for  it  and 
to  test  the  cement  directly  as  to  its  soundness  and  expansibility 
in  water.  (See  p.  124). 


46    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

In  the  rotary  and  Hoffmann  kilns  used  for  cement  manu- 
facture there  is  generally  sufficient  draught  for  the  complete 
removal  of  all  the  carbon  dioxide  liberated,  but  in  some  of  the 
shaft  kilns  a  small  but  recognisable  proportion  of  undecomposed 
(or  re-formed)  calcium  carbonate  is  usually  present. 

The  impurities  present  in  the  chalk  or  limestone  will  behave 
— so  far  as  any  chemical  changes  are  concerned — like  the 
similar  constituents  of  the  clays  or  like  calcium  carbonate. 
Thus,  magnesium  carbonate  is  decomposed,  forming  free 
magnesia  and  carbon  dioxide. 

Chemical  Reactions  between  Clay  and  Lime. 

The  most  important  chemical  changes  which  occur  in  the 
manufacture  of  cements  are  not  the  direct  chemical  action  of 
heat  on  the  various  substances  used,  each  being  considered 
separately,  but  are  due  to  the  various  reactions  of  the  various 
substances  upon  each  other.  Thus,  either  a  pure  clay  or  lime, 
when  heated  separately,  is  quite  infusible  at  all  industrial 
temperatures,  but  when  a  mixture  of  clay  and  lime  in  suitable 
proportions  is  heated,  it  melts  at  a  temperature  of  1400°  C.  or 
lower,  and  the  product  is  entirely  different  in  chemical  and 
physical  properties  from  the  original  mixture.  Owing  to  the 
variety  of  substances  present  in  natural  clays  and  chalk  or 
limestone,  the  various  reactions  of  these  upon  each  other  are 
extremely  difficult  to  study.  Some  of  the  reactions  at  present 
regarded  as  of  minor  importance  may  be  proved,  later,  to  be  of 
greater  significance  ;  at  the  moment  of  writing,  however,  the 
following  are  considered  to  be  the  chief  reactions  which  occur  : 

Those  portions  of  the  contents  of  a  kiln  which  have  attained 
a  temperature  of  1000°  C.  or  above  will  not  consist  of  the 
original  clay  and  chalk  or  limestone  fed  into  the  kiln,  but  of  a 
mixture  of  "  calcined  clay  "  1  and  free  lime,  together  with 
such  free  silica  (including  tridymite)  and  other  "  calcined 
minerals  "  as  may  be  present  adventitiously.  At  the  tempera- 
ture mentioned  some  amount  of  fusion  will  have  taken  place, 
particularly  among  the  "  calcined  minerals,"  and  the  glassy 

1  It  is  clear  that  with  the  present  divergence  of  opinion  as  to  the  nature  of  this 
"  calcined  clay"  (p.  43)  it  is  inadvisable  to  regard  it  definitely  as  either  a  mixture 
of  free  silica  and  alumina  or  as  an  aluminosilicic  complex. 


REACTIONS  BETWEEN  CLAY   AND   LIME        47 

substances  so  produced  will  bring  the  particles  of  lime  and 
"  calcined  clay  "  into  such  intimate  contact  with  each  other 
that  various  reactions  will  commence.  Indeed,  one  object  of 
the  fine  grinding  and  thorough  mixing  of  the  raw  materials  is 
to  secure  the  most  intimate  contact  possible  between  the  various 
materials. 

That  no  fusion  is  necessary  is  clearly  observable  if  a  mixture 
of  pure  china  clay  and  pure  lime  is  heated  in  a  Doelter's 
microscope,  when  it  will  be  found  that  no  appreciable  fusion 
occurs  until  a  temperature  of  1300°  C.  is  reached,  though  the 
complete  solubility  of  the  product  heated  to  a  lower  tempera- 
ture will  show  that  combination  of  the  two  substances  has 
occurred.  Moreover,  as  shown  later,  J.  W.  Cobb  has  con- 
clusively proved  that  reactions  between  lime,  silica  and 
alumina  can  occur  with  the  production  of  a  mass  completely 
soluble  in  hydrochloric  acid  at  temperatures  far  below  that 
at  which  even  partial  fusion  takes  place.  The  partial  fusion 
which  occurs,  with  some  clays,  at  a  temperature  of  1000°  C.  is 
due  to  the  impurities  present,  and  has  no  essential  connection 
with  the  progress  of  the  main  reaction  whereby  cement  clinker 
is  formed,  though  it  may  have  a  physical  effect  in  increasing 
the  rapidity  of  the  reaction  by  bringing  the  particles  into 
more  intimate  contact  with  each  other. 

If  the  "  calcined  clay  "  is  regarded  as  a  simple  mixture  of 
silica  and  alumina,  both  in  the  free  state,  the  chief  action  of 
the  lime  will  be  to  form  : — 

(a)  calcium  silicates  (xCaO,ySi02) 

(b)  calcium  aluminates  (xCa 

(c)  calcium  aluminosilicates 

These  substances  will  be  mixed  together  in  proportions 
depending  on  the  relative  amounts  of  lime  and  "  clay,"  on 
the  duration  of  the  heating,  and  on  other  conditions  required 
for  the  formation  of  each  of  these  classes  of  substances. 

From  a  simple  mixture  of  lime,  silica  and  alumina,  it  is  most 
natural  to  suppose  that  the  product  would  consist  largely  of 
a  mixture  of  one  or  more  calcium  silicates  with  one  or  more 
calcium  aluminates,  and  that  the  proportion  of  calcium 
aluminosilicates  would  be  very  small.  The  apparent  simplicity 


48    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

of  this  arrangement  has  made  many  investigators  content  to 
regard  cements  as  mixtures  of  simple  binary  compounds,  and 
especially  as  solid  solutions  of  a  calcium  aluminate  in  a  calcium 
silicate.  This  theory  of  solution  is,  indeed,  so  popular  that 
very  few  writers  realise  the  peculiarly  slender  foundations  on 
which  it  is  based  and  the  enormous  lacunce  between  the  assump- 
tions made  as  to  the  theoretically  possible  existence  of  certain 
substances  and  the  complete  failure  to  produce  these  substances 
under  the  conditions  existing  in  the  manufacture  of  cements. 

If,  on  the  contrary,  the  "  calcined  clay  "  is  considered  to  be 
an  aluminosilicic  complex  or  anhydride,  it  is  more  probable 
that  the  greater  part  of  the  product  would  consist  of  ternary 
compounds,  i.e.,  calcium  aluminosilicates,  and  that  the  pro- 
portion of  binary  calcium  silicates  and  calcium  aluminates 
would  be  small.  The  presence  of  all  three  classes  of  substance x 
is,  of  course,  quite  probable,  whichever  theory  as  to  the  chemical 
constitution  of  "  calcined  clay  "  is  adopted  ;  it  is  only  the 
relative  proportions  of  each  which  is  important.  Again,  it  is 
quite  possible  that  the  action  of  the  lime  on  any  aluminosilicic 
anhydrides  present  may  result  in  a  decomposition  of  the 
complex  and  the  consequent  formation  of  simple  binary 
silicates  and  aluminates,  though  there  are  serious  objections 
to  the  view  that  this  decomposition  occurs  to  any  great  extent. 

Three  entirely  different  series  of  methods  of  investigation 
have  been  adopted  in  studying  the  constitution  of  cements. 
The  first  of  these  comprises  analytical  methods  applied  to  the 
cements  themselves  and  including  the  study  of  the  physical 
properties  as  well  as  their  behaviour  towards  chemical  reagents. 
The  second  method  of  investigation  comprises  synthetic 
methods  of  research  which  consist  essentially  in  endeavour- 
ing to  prepare  the  apparent  or  presupposed  constituents  of 
cements,  and  in  comparing  the  properties  of  these  synthetic 
products  with  cements  prepared  for  purposes  of  commerce. 
The  third  method  consists  in  observing  the  reactions  which 
occur  when  the  raw  materials  are  heated  or  when  the  clinker 
is  cooled.  In  this  connexion  it  is  important  to  observe  that 
great  care  must  be  taken  to  avoid  overheating,  with  the 

1  This  is  due  to  the  use  of  impure  materials. 


METHODS   OF   INVESTIGATION  49 

resultant  fusion  of  the  clinker,  for  this  produces  substances 
which  do  not  necessarily  exist  in  ordinary  cement  clinker. 
Dittler  and  Herold  go  so  far  as  to  state  that  observations  of 
what  occurs  when  well-made  and  carefully-burned  cement 
clinkers  are  allowed  to  cool,  can  never  show  the  constitution 
of  the  clinker  satisfactorily  as  the  formation  of  crystals  does 
not  take  place  like  that  in  most  other  systems  of  two  or  more 
components,  for  instead  of  the  substances  in  excess  separating 
first,  it  is  the  substance  which  has  the  greatest  rate  of  crystal- 
lisation which  first  becomes  crystalline.  The  formation  of  the 
various  compounds  does  not  occur  during  the  cooling,  but 
during  the  heating,  the  fusion  and  crystallisation  occurring 
simultaneously.  For  these  reasons  the  results  of  observations 
made  on  cooling  clinker  should  be  accepted  with  great  reserve. 

In  studying  what  occurs  when  the  raw  materials  are  heated 
under  conditions  where  the  reactions  can  be  observed,  the 
changes  in  the  electrical  conductivity  of  the  mixture  are  of 
great  value.  It  is  well  known  that  the  extent  to  which  a 
substance  will  conduct  electricity  is  a  measure  of  its  dissocia- 
tion into  ions,  and  it  is  an  important  fact  that  when  clays  and 
cements  are  heated  above  700°  C.  their  electrical  conductivity 
increases  rapidly  with  increasing  temperature  to  900°  C.,  after 
which  it  is  rather  slower  ;  at  temperatures  of  1400°  C.  to 
1600°  C.  it  is  as  high  as  that  of  aqueous  solutions  of  corre- 
sponding salts.  This  behaviour  implies  that  at  the  highest 
temperature  reached  in  rotary  kilns,  cement  clinker  is  com- 
pletely dissociated  into  its  separate  ions,  though  the  constitu- 
tion of  these  ions  has  not  yet  been  ascertained. 

If  Asch's  theory  (p.  55)  is  correct,  these  ions  would  be 
Ca  and  6Al203l2Si02.  No  other  published  theory  explains  in 
a  simple  manner  what  ions  are  produced  by  this  dissociation. 

Each  of  the  lines  of  investigation  mentioned  above  is 
important  and  neither  is  reliable  without  the  others,  yet  so 
difficult  is  the  study  of  cements  and  so  great  is  the  influence 
of  the  theories  of  the  earlier  investigators  that,  in  spite  of  the 
enormous  amount  of  evidence  available,  it  is  difficult  to  reach 
conclusions  which  will  be  accepted  by  all  chemists  interested 
in  cements. 

Thus,  a  microscopic  examination  of  a  thin  section  of  clinker, 

c,  E 


50  CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 


or  preferably  a  similar  study  of  a  piece  of  first-class  clinker 
which  has  been  polished  and  then  etched  with  very  dilute  acid 
or  water,  shows  that  such  clinker  is  chiefly  composed  of  a 
single  crystalline  constituent,  the  particles  of  which  are 
separated  by  a  much  smaller  quantity  of  other  material. 
Clinker  from  a  rotary  kiln  usually  contains  less  intercrystalline 
matter  than  that  obtained  from  stationary  kilns,  as  the  burning 
is  more  efficient  in  the  former.  As  the  crystals  of  the  principal 
constituent  form  so  large  a  proportion  of  the  whole  material 

in  really  well-made 
clinker,  it  is  not  un- 
reasonable to  sup- 
pose that  these 
crystals  represent  the 
really  essential  con- 
stituent, and  that  the 
remaining  materials 
are  of  an  entirely 
adventitious  charac- 
ter and  are  due  to 
impurities  in  the  raw 
materials  or  to  im- 
perfections in  the 
processes  of  manu- 
facture. 

Le  Chatelier  and 
Tornebohm  were 
among  the  first  in- 
vestigators to  use 
the  microscope  in  investigating  the  nature  of  cement  clinker 
in  the  manner  described  above.  They  observed  four  different 
kinds  of  crystals,  for  which  Tornebohm  proposed  the  names 
alite,  belite,  celite  and  /elite,  respectively.  By  far  the  most 
important  of  these  are  alite  and  celite,  particularly  the 
former,  which  constitutes  the  principal  portion  of  the 
material,  the  celite  (with  occasionally  a  little  belite  and  felite) 
forming  a  filling  material  or  matrix  between  the  grains  of 
alite. 

1  Courtesy  of  C.  H.  Desch,  Esq. 


FIG.  5. — Cement  Clinker  x  180  diams.1 

(Lightly  etched  with  very  dilute  hydrochloric 
acid.) 


ALITE,    BELITE,    CELITE   AND   FELITE  51 

Belite  resembles  alite  in  some  respects,  but  is  a  dirty  green 
colour,  is  characteristically  striated,  and  gives  brilliant  inter- 
ference colours.  It  has  never  been  isolated  in  a  crystalline 
form  nor  in  a  state  sufficiently  pure  for  analysis,  but  is  generally 
understood  to  be  calcium  ortho-silicate,  2CaOSi02. 

Calcium  ortho-silicate,  prepared  synthetically  by  heating  a 
mixture  of  lime  and  silica  in  equivalent  proportions  to  1150°  C., 
melts  at  2074°  C.  It  is  almost  devoid  of  hydraulicity  and 
rapidly  falls  to  powder  on  exposure  to  air.  According  to  Day 
and  Shepherd,  there  are  three  calcium  ortho-silicates,  depending 
on  the  temperature  of  the  material,  the  a-form,  which  crystal- 
lises in  monoclinic  prisms  with  a  hardness  of  5  to  6  on  Moh's 
scale  and  is  only  stable  above  1410°  C.  ;  the  /3-form,  which  is 
produced  when  the  a-form  is  cooled  from  1410°  C.  to  about 
675°  C.,  and  is  ortho-rhombic  with  a  specific  gravity  of  3-27  ; 
and  the  y-form,  which  is  produced  by  cooling  to  temperatures 
below  675°  C.  The  y-form  has  a  specific  gravity  of  2-97,  so 
that  its  formation  is  accompanied  by  an  increase  in  volume 
which  accounts  for  the  material  disintegrating  as  the  rapidly- 
cooled  a-form  is  slowly  converted  into  the  /3-ortho-silicate. 

Various  attempts  have  been  made  to  explain  the  difference 
between  the  slowly  cooled,  or  y-ortho-silicate,  and  the  hy- 
draulic, or  a-ortho-silicate,  obtained  by  suddenly  quenching 
the  molten  mass,  by  representing  them  as  of  different  molecular 
arrangements.  Such  attempts  are,  however,  founded  on  data 
which  are  far  too  slight  to  justify  the  use  of  different  structural 
formulae. 

It  should  be  observed  that  the  calcium  ortho-silicates  obtained 
by  Day  and  Shepherd  were  produced  by  heating  mixtures  of 
lime  and  silica  to  complete  fusion  and  then  allowing  the  molten 
mass  to  cool  to  various  temperatures.  Slags  are  subjected  to 
this  treatment,  so  that  the  work  of  these  investigators  is 
valuable  when  applied  to  slags ;  but  in  the  manufacture  of 
other  cements  complete  fusion  is  never  reached,  and  the 
conditions  are  so  entirely  different  as  not  to  warrant  the 
application  of  these  investigations  to  Portland  cement  (p.  49). 

Celite  is  recognised  by  its  deep  brownish-orange  colour.  It 
is  of  lower  fusing  point  than  alite  and  gives  brilliant  colours 
when  examined  between  crossed  nicol  prisms,  Richardson 

E  2 


52    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

claims  to  have  identified  celite  with  dicalcium  aluminate 
(2CaOAl203)  in  solution  in  dicalcium  silicate  (2CaOSi02). 

Celite  has  never  been  isolated  and  examined  apart  from  the 
other  constituents  of  cement.  Its  composition  is,  therefore, 
entirely  problematical,  and  definite  statements  with  regard  to 
it  should  be  accepted  with  reserve. 

Felite  forms  colourless  rhombic  crystals  in  some  partially 
decomposed  blast-furnace  slags,  but  is  by  no  means  common. 
Its  composition  is  not  accurately  known,  though  Kappen  and 
others  believe  it  to  be  a  non-hydraulic  form  of  calcium  ortho- 
silicate.  Some  chemists  maintain  that  it  is  a  magnesium  ortho- 
silicate,  or  a  double  silicate  of  magnesium  and  calcium. 

Alite  crystals  belong  to  the  rhombic  system,  and  tend  to 
assume  hexagonal  forms,  but  their  properties  are  by  no  means 
clearly  established.  C.  Richardson  claims  to  have  identified 
alite  as  a  solid  solution  of  tricalcium  aluminate  (3CaOAl20^)  in 
tricalcium  silicate  (3CaOSi02).  Such  a  solid  solution  cannot 
exist  in  the  form  of  such  definite  crystals  as  those  generally 
recognised  as  alite.  Le  Chatelier  has  found  that  certain 
grappiers  (p.  34)  consist  almost  entirely  of  alite,  and  contain  :— 

Per  cent. 

Lime    .          ,          .          .          .          .  66-0 

Silica    .          .'.        ?          .          .          .  26-0 

Alumina      .  .          .          .         V         .~  3-5 

Ferrous  oxide  (FeO)        .  .       •.  •       .   "  1-0 

Water,  etc.    .          .       "" .   "       .          .  3-5 
CaO  :  Si02  ratio  =  2-75  :  1  mols. 

This  is,  however,  far  too  low  in  alumina  to  be  comparable 
to  the  best  Portland  cement. 

Le  Chatelier  regarded  the  alumina  and  water  in  cement  as 
impurities,  and  concentrated  his  attention  exclusively  on  the 
lime  and  silica  present.  He  therefore  endeavoured  to  prepare 
a  synthetic  cement  in  which  the  ratio  of  CaO  :  8i02  =3:1, 
on  the  assumption  that  the  lower  ratio,  2-75  :  1,  in  grappiers 
and  in  some  of  the  best  Portland  cement  is  due  to  impurities. 
He  found  that  a  calcined  mixture  containing  lime  and  silica 
in  the  proportion  of  3  molecules  to  1  (3Ca08i02)  remained 


TRICALCIUM   SILICATE  53 

constant  in  volume  on  setting,  but  hardened  remarkably 
slowly.  The  brothers  Newberry  confirmed  this,  and  showed 
that  with  even  a  slight  excess  of  lime  (SJCaO  .  Si02)  the  product 
did  not  form  a  sound  cement,  but  cracked  when  kept  under 
water.  Le  Chatelier  also  found  that  commercial  cements  with 
only  sufficient  lime  to  correspond  to  a  ratio  of  2-5CaOSi0.2 
were  inferior  in  strength,  liable  to  disintegrate  and  deficient 
in  alite. 

Nevertheless,  the  evidence  is  almost  conclusive  against  the 
existence  of  3CaOSi0.2  in  notable  quantity  in  cements,  or  even 
of  the  possibility  of  its  formation  by  heating  pure  lime  and 
silica  in  suitable  proportions.  Day,  Shepherd  and  Wright. have 
found  that  specimens  which  were  thought  to  be  the  synthetic 
trisilicate  were  really  composed  of  an  intimate  mixture  of 
lime  and  calcium  ortho-silicate  crystals,  which  may  easily  be 
mistaken  for  a  homogeneous  substance. 

Some  chemists  have  suggested  that  the  microscopical 
appearance  of  alite  crystals  implies  that  profound  modifications 
have  been  induced  in  consequence  of  the  presence  of  adven- 
titious substances  in  the  cement,  as  a  result  of  which  the 
3CaOSi0.2  has  been  rendered  stable.  Such  a  supposition  is 
purely  speculative  and  unnecessary.  The  repeated  unsuccess- 
ful attempts  by  such  competent  authorities  as  Le  Chatelier 
and  others  to  produce  a  really  satisfactory  cement  from  pure 
lime  and  silica  indicate  that  some  constituent  other  than  lime 
and  silica  is  essential.  The  clearly  recognisable  differences 
between  cements  made  from  basic  slag  and  limestone  and  those 
made  from  clay  and  limestone  also  indicate  that  the  particular 
state  of  combination,  or  the  existence  in  the  free  state,  of  the 
alumina  and  silica  present  is  of  great  importance  as  regards 
the  structure  of  the  resultant  cement.  The  constancy  of  the 
ratio  CaO  :  8i02  =3:1  appears  to  be  merely  a  coincidence 
and  not  to  indicate  the  whole  of  the  facts,  for  this  ratio  would 
be  the  same  in  a  compound  with  the  formula  3xCaO  .  yAl.20.3 . 
xSi02,  but  the  properties  of  such  a  compound  would  be  entirely 
different  from  those  of  SxCaO  .  xSi02  on  account  of  the  alumina 
present.  It  is  precisely  because  investigators  have  been 
obsessed  with  the  idea  of  the  existence  of  3CaO  .  Si0.2  in  cements 
that  most  of  them  have  overlooked  the  possible  existence  of 


54    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

calcium  alumino-silicates,  whilst  most  of  those  who  have 
recognised  the  possibility  of  such  triple  compounds  have  not 
pursued  their  researches  far  enough  to  prove  their  case.  It  is 
one  of  the  curiosities  of  modern  chemistry  that  so  many 
investigators  have  so  willingly  accepted  statements  as  to  the 
existence  of  certain  silicates  and  aluminates  in  spite  of  the 
fact  that  these  substances  cannot  be  prepared  under  conditions 
met  with  in  the  manufacture  of  cements.  Yet  these  considera- 
tions have  been  almost  entirely  overlooked  by  those  engaged 
in  investigations  of  the  chemical  constituents  of  cements. 

Thus,  to  account  for  the  presence  of  alumina  in  all  satis- 
factory cements,  various  investigators  have  prepared  calcium 
aluminates  corresponding  to  the  ratio  2CaO  .  Al.2O.^  and 
finding  that  these  set  very  rapidly,  producing  cements  of 
constant  volume  when  set  and  of  good  hardening  properties, 
they  at  once  assumed  that  such  binary  aluminates  are  present 
in  Portland  cement  in  simple  admixture  with  the  alite  already 
mentioned.  Unfortunately,  no  one  has  yet  been  able  to  isolate 
such  calcium  aluminates  in  appreciable  quantities  from 
commercially  valuable  cements,  and  as  calcium  aluminates, 
when  pure,  turn  an  alcoholic  solution  of  phenolphthalein  red, 
their  presence  in  Portland  cement  would  be  readily  detected 
by  this  indicator.  Well  made  and  freshly  burned  Portland 
cements  do  not,  however,  produce  any  colour  with  this  solution, 
so  that  it  is  improbable  that  free  calcium  aluminates  occur  in 
these  cements.  Thus,  although  the  theory  that  cements  are 
composed  of  simple  mixtures  of  substances,  such  as  3CaO  .  Si02 
and  2CaO  .  ALO^,  is  in  some  respects  simple  and  convenient, 
its  disadvantages  are  very  great. 

Some  of  the  largest  alite  crystals  yet  obtained  were  produced 
by  Schmidt  and  Unger  by  heating  Portland  cement  to  complete 
fusion  in  an  electric  arc.  The  whole  mass  became  crystalline 
on  cooling,  and  consisted  of  a  largely  preponderating  proportion 
of  crystals  all  of  one  kind,  together  with  a  microcrystalline 
aggregate  of  a  different  substance,  possibly  celite,  but  more 
probably  a  heterogeneous  mixture  of  crystals  containing  all 
the  "  impurities  "  in  the  cement.  The  largest  and  most 
perfect  crystals  proved  to  be  identical  in  physical  characters 
with  alite.  On  analysis  they  were  found  to  contain  : — 


TRICALCIUM  SILICATE 


55 


(a) 


Lime  ......  67-43  per  cent. 

Silica  .          .          .          .          .  23-43 

Alumina      .          .          .          .          .  3-78         „ 

Ferrous  oxide       .          .          .          .  2-32         ,, 

Magnesia     .          .          .          .          .  2-44         ,, 

Water,  etc.     .       .          .          .          .          0-60 

These  results  agree  sufficiently  closely  with  those  found  by 
Le  Chatelier  (p.  52),  but  are  lower  in  alumina  than  are  the 
best  Portland  cements. 

W.  and  D.  Asch  have  shown  that,  in  all  probability,  there 
is  no  single  substance  —  alite  —  which  is  the  essential  constituent 
of  all  Portland  cements,  but  that  these  materials  contain  one 
or  more  calcium  alumino-silicates  of  a  highly  complex  struc- 
tural composition.  They  consider  that  Portland  cements  are 
basic  salts  of  these  aluminosilicic  acids,  and  that  they 
consist  of  a  series  of  hexagonally  or  pentagonally  arranged 
groups  of  silicon  and  aluminium  atoms  to  which  are  attached 
a  number  of  side  chains,  the  latter  containing  the  greater  part 
of  the  base. 

In  Great  Britain,  the  clays  and  shales  generally  used  for 
cement-making  are  highly  siliceous,  and  probably  have  a  con- 
stitution corresponding  to  (a)  HlsAlQSil2042,  or  (b)  H1&Al^8iu0^f 
though  other  types  are  possible.  In  accordance  with  Asch's 
theory  these  typical  clays  have  the  following  constitutional 
formulae  l  :— 

(OH)z  OH  (OH)2 


0 


o 


(OH),= 


0 

0           C 

) 

0 

0 

Si—O—Al 

Al—O—Si 

\ 

(OH) 

\\ 
0  0        ( 

, 

( 

1 

06 

> 

!'= 

1  / 

\l 

i 

Si—O—Al 

Al—O—Si                Si  = 

(OH). 

/ 

\ 

/ 

/ 

\ 

/ 

o 

0           ( 

9 

0 

0 

\Al/ 

1 

\Si/ 

/ 

OH 

(OR), 

(OH) 

1  The  lime,  magnesia  and  alkalies  present  in  the  clays  replace  equivalent 
hydroxyl  (OH]  groups  and  the  iron  compounds  as  described  on  p.  74,  but  for 
clearness  the  formulae  shown  are  those  for  pure  clays  of  the  same  type. 


56    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 


O          0 


O 


o, 
o- 


o 
^ 


o 

£1 


I 


\ 


o        o 


^ 


O 


$' 


These  may  be  represented  more  simply,  as 


I 

/\ 

8i\Al 

\/ 
I 


Si 


=(    Si 


Al 


(a) 


(6) 


THE   ALUMINO-SILICATE   THEORY 


57 


On  heating  with  lime,  combination  occurs  which,  in  the  case 
of  formula  (a),  may  be  represented  by  the  following  equation  :— 

HisAlvSiwOu  +  38<7aO  =  H£a&Al$iuf)n  +  8#20 

water  being  liberated  owing  to  the  hydroxyl  (OH)  groups 
which  give  the  clay  its  acid  character  being  replaced  by  basic 
groups  and  forming  a  cement  with  the  structural  formula  on 
p.  58,  which  may  be  represented  in  an  abbreviated  form  by  :— 


5CaO.CaO.5CaO. 
1 


4CaO  — 
4CaO  = 


=  4CaO 
=  4CaO 


5CaOCa05CaO 

Clays  of  type  (b)  form  cements  with  formulae  similar  to  the 
second  one  on  p.  59. 

According  to  this  theory,  there  is  an  extremely  large  number 
of  calcium  alumino-silicates  of  very  similar  percentage  com- 
position, yet  having  noticeably  different  chemical  structures. 
This  number  is  indefinitely  increased  by  the  presence  of 
magnesia,  potash,  soda  and  iron  oxide  in  the  raw  materials 
from  which  cements  are  made,  these  oxides  forming  still  more 
complex  groupings  around  the  central  hexagon  or  pentagon 
rings.  Any  potash  or  soda  present  may  replace  part  or  all  of 
the  CaO  attached  to  the  ^4Z-ring ;  any  magnesia  will  usually 
replace  one  or  more  of  the  CaO  molecules  at  the  sides  of  the 
formula.  These  complications  have,  however,  been  omitted 
for  the  sake  of  clearness. 

The  following  are  typical  examples  of  Portland  cements  : — 

(2)  (2)  OK  (2)  (2) 


4°= 


+  0'5Na(K)CO< 


ONa  4° 

.  3lCaO  .  MgO  . 


c 

o 
o 
e 

6 


58    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

/'0\ 


O 

o 

js 


o       o 

0_ 


^ O CQ 

1^^4 


<S-_0--^ 


/*\ 


-o — ' 


o 


5 


o 
6 


o 
6 


THE   ALUMINO-SILICATE   THEORY 

5°    OK  5° 


59 


4o_ 
4o_ 


,=4° 
Si  +  0-SNaCl 

_4o 


5°   OK    5° 
=  3oCaO  .  MgO  .  K^O  . 

where  each  hexagonal  ring  represents  6Si0.2  or  3A12O3,  and  the 
numbers  indicate  the  corresponding  number  of  molecules  of 
CaO  or  MgO  which  have  replaced  OH  groups  in  the  clay.  A 
somewhat  different  type  of  cement  is 


39<7aO  . 

in  which  the  hexagonal  ring  represents  3A1203  and  each 
pentagon  represents  5Si0.2,  the  numbers  having  the  same 
meaning  as  before. 

If  this  theory  is  correct,  what  occurs  when  a  mixture 
of  clay  and  calcium  carbonate  is  heated  under  the  conditions 
usual  in  the  manufacture  of  Portland  cement  is  that  the 
hydroxyl  groups  attached  to  the  silicon  atoms  in  the  clay  are 
replaced  by  anhydrobasic  groups  derived  from  the  lime.  This 
implies  that  on  heating  clays  the  molecule  is  not  broken  up 
into  its  constituent  oxides,  but  that  a  single  compound  is 
formed  (p.  41),  or  the  material  is  dissociated  into  its  respective 


60    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 


ions.  These  ions  may,  at  a  later  stage  in  the  use  of  the  cement, 
re-act  as  though  a  single  compound  was  present.  The  loss 
of  resistance  to,  or  conversely  the  increase  of  conductivity  of, 
electricity  suffered  by  the  raw  materials  in  the  manufacture  of 
Portland  cements  is  shown  in  Fig.  6,  which  represents  the 
results  obtained  by  E.  Dittler  and  K.  Herold  on  (1)  a  dry  marl, 
(2)  another  dry  marl,  (3)  a  cement  mix  of  chalk  and  marl  made 
by  the  dry  method,  (4)  a  cement  mix  of  limestone  and  clay 

made  by  the  wet 
method,  and  (5)  a 
mixture  of  pure  cal- 
cium carbonate  and 
kaolin  (china  clay). 
The  general  simi- 
larity of  the  curves 
is  very  striking,  and 
it  appears  highly 
probable  that  the 
differences  observ- 
able —  which  are 
greatest  at  the  lower 
temperatures  -  -  are 
chiefly  due  to  the 
variations  in  the 
composition  of  the 
materials  rather  than 
to  any  differences  in 
the  reactions  which 
occur. 

(Zentr.    Chem.    Anal.    Hydraul. 
have  found    that  the  electrical 


800  850  900   050  1000  1050  1100  1150  1200  12501300  1350  WHf 

FIG.  6. — Electrical  Conductivity  of  Kaw 
Cement  Mixes. 


E.  Dittler  and  L.  Jesser 
Zemente,  1910,  pp.  71—78) 
conductivity  increases  gradually  until  a  temperature  of  about 
1375°  C.  is  reached,  when  there  is  a  break  in  the  graph  repre- 
senting it  accompanied  by  an  endothermic  reaction  and  slight 
fusion,  and  when  the  temperature  reaches  1425  to  1450°  C.  an 
exothermic  reaction  occurs  suddenly  with  the  immediate 
formation  of  crystals  of  alite  (?)  with  a  little  celite.  The 
change  in  conductivity  and  formation  of  the  alite  (?)  is  shown 
in  Fig.  7,  which  is  the  resistance  graph  of  a  clay-limestone 


ELECTRICAL   CONDUCTIVITY   OF   CEMENTS      61 


cement    maintained    for    three    hours    at    a    temperature   of 
1430°  C. 

Such  a  break  in  the  conductivity  curve  of  many  minerals  is 
well  known  to  coincide  with  their  melting  points,  and  clearly 
points  to  the  formation  of  a  crystalline  compound  of  definite 
composition  and  with  a  sharply  marked  melting  point  of 
1425°  C.  This  result  is  in  complete  opposition  to  the  view 
that  cements  are  composed  of  a  mixture  of  tricalcium  silicate 
and  dicalcium  aluminate,  and  shows  that  a  cement  clinker 

is  a    definite          

chemical  indi- 
vidual and  not  Ohms 
a  solid  solution 
of  two  or  more 
substances.  In 
this  respect  it 
confirms  Asch's 
theory  that  the 
chief  constitu- 
ent of  cements 
is  a  definite 
calcium  alu- 
mino- silicate. 

If  alite  has 
the  definite 
c  o  mposition 
such  as  those 
assigned  to  the 
chief  con- 


700 


600 


500 


400 


300 


200 


100 


190       210        230      250       270       290      310       330    mm-. 
FIG.  7. — Electrical  Conductivity  of  Clay -lime 
Mixture. 


stituent  in  cements  by  W.  and  D.  Asch — and  there  seems 
little  reason  to  doubt  that  its  composition  is  quite  definite 
for  any  given  cement,  whatever  may  be  the  arrangement 
of  the  atoms  within  the  molecule  —  it  is  clear  that  the 
best  qualities  of  cement  will  be  those  in  which  the  composition 
most  closely  resembles  that  of  the  particular  alumino-silicate 
(alite  ?)  present  in  the  largest  proportion.  It  has  long  been 
recognised  that  very  small  variations  in  the  percentage  of  lime 
cause  serious  differences  in  the  strength  and  properties  of  the 
cement,  and  that  the  limits  of  composition  of  the  mixture  of 


62    CHEMI€AL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

raw  materials  are  very  narrow.  In  endeavouring  to  obtain 
some  simple  guide  of  the  limits  of  composition  of  cements,  it 
has  been  customary  to  follow  Le  Chatelier's  assumption  that 
Portland  cements  are  composed  of  3CaOSi0.2  and  3CaOAl.20.j 
and  Newberry 's  assumption  that  they  are  composed  of 
3CaOSi0.2  and  2CaOAL20$.  On  these  assumptions  a  cement 
will  consist  of  a  molecules  of  3CaOSi0.2,  and  b  molecules  of 
either  3CaOAW.3  or  2CaOAl.20^.  From  this  it  follows  that 
the  ratio  between  the  lime  and  (silica  +  alumina)  in  cements 
should  remain  constant,  and  the  ideal  proportion  of  these  oxides 
would  then  be — 
(1)  according  to  Le  Chatelier  : 

molecules  CaO 


molecules  Si02  +  molecules 
(2)  according  to  Newberry  : 

3a  -j-  26  molecules  CaO 


a  molecules  SiO^  +  b  molecules 


=    3 


In  spite  of  the  fact  that  ^CaOAl203  has  not  been  proved  to 
exist  in  cements  and  that  the  constant  3  is  too  high,  the  equation 
suggested  by  Newberry  has  not  been  adopted,  and  Le  Chatelier's 
is  still  regarded  in  many  quarters  as  indicating  the  maximum 
proportion  of  lime  which  can  exist  in  a  sound  cement.  In  the 
British  standard  specification  for  Portland  cement  a  somewhat 
lower  constant  is  given,  viz.  : — 

molecules  CaO  —2- 85 


molecules  $*Oa  +  molecules  Al20t 

This  demands  a  somewhat  lower  lime  content  than  that 
suggested  by  Le  Chatelier. 

Where  the  use   of  molecular  equivalents  is  inconvenient, 
the  limit  is  expressed  as 

%  lime  =  2-8  X  %  silica  +  1-1  X  %  alumina 

The  exact  proportion  of  lime  which  can  be  used  will  depend  on 
the  care  and  skill  used  in  the  manufacture  of  the  cement.  If 
the  raw  materials  are  extremely  finely  ground,  thoroughly 
mixed  and  properly  burned,  a  higher  proportion  of  lime  may  be 


FORMULA   FOR   COMPOSITION 


63 


present  than  in  a  less  skilfully  prepared  cement.     The  larger 
the  proportion  of  lime  present,  combined  as  alumino-silicate, 


w 

/ 

, 

.V 

7 

56 

^ 

/ 

/ 

/ 

52 

/ 

7 

50 

/ 

f 

48 

/ 

4i 

/ 

<ib 

<<;V 

V/ 

v4 

/ 

X)  / 

^/ 

<^ 

J3 

^ 

/ 

^ 

/ 

JV. 

7 

Jb 

4^Y> 

inf 

xce 

s  of 

(  im 

e 

- 

^ 

/ 

£ 

^^ 

/•  <$> 

^i 

V 

A) 

s 

7 

j/ 

f 

,b- 

I 

/ 

tjf. 

W 

/ 

/. 

X" 

x\^ 

/ 

/^  ^ 

!• 

/ 

/ 

x^y 

~7 

/ 

/ 

/ 

/ 

^7* 

/ 

/ 

/ 

/ 

., 

/ 

./ 

/ 

> 

t\rp 

a  ot 

E 

•S5 

ES 

//("<- 

/ 

"~? 

/ 

/ 

/ 

/* 

d 

/ 

/  * 

/'O 

/ 

/ 

/ 

J 

'    ^t 

? 

^ 

'/ 

i 

i- 

I      2345     6     7      8    9     10    II    12    13    14    IS     16    17    18    13   20 

mols.  Si  02 

FIG.  8. — Empirical  Limits  of  Composition  of  Portland  Cements. 

the  stronger  will  be  the  cement.     Lime  in  other  forms  is  less 
valuable,  and  uncombined  or  free  lime  is  dangerous. 

As  the   proportions   of   lime,   silica   and   alumina   all   vary 
in  different  samples  of  commercial  cement,  it  is  convenient, 


64    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

in  classifying  samples,  to  express  the  proportions  in  molecular 
equivalents,  the  number  of  molecules  of  alumina  being  taken 
as  unity,  thus  — 

UCaO  •  AW-. 


In  this  way  one  of  the  three  variables  is  removed  and  the 
remaining  two  may  be  plotted  on  a  chart  as  ordinates  and 
abscissae,  respectively.  Thus,  accepting  the  limits  in  the 
British  standard  specification  given  above,  the  molecular 
ratio  of  each  of  the  oxides  is  — 

2-85  (a  +  b)  molecules  CaO 
a  molecules  Si02 
b  molecules  Al<£)3 

and  by  taking  6=1,  the  maximum  permissible  limit  of  lime  = 
2-85  (a  -{-  1)  molecules  to  each  a  molecules  of  silica  and  1 
molecule  of  alumina.  The  disadvantages  of  assuming  the 
existence  of  3CaOSi02  and  2CaOAl2Os  in  cements  are,  in  this 
way,  avoided,  and  the  influence  of  each  of  the  constituent 
oxides  is  most  easily  perceived.  Fig.  8  is  a  chart  of  this 
kind,  based  on  the  ratios 


xCaO  ySi02  1-00 

in  which  the  various  values  of  x  are  plotted  on  the  ordinate  and 
those  of  y  on  the  abscissae.  The  value  for  alumina  being  made 
constant  it  does  not  require  to  be  plotted.  In  order  to  show 
the  limits  within  which  commercially  useful  cements  lie,  the 
curve  giving  the  maximum  limits  for  lime  recognised  by  the 
British  standard  specification  based  on  the  ratios 

2-85  (a  +  1)  CaO,  aSi02,  1-00  A1203 

is  shown.  The  lowest  proportions  of  lime  generally  recognised 
as  permissible  is  that  given  in  the  hydraulic  modulus,  a  term 
introduced  by  W.  Michaelis,  and  adopted  in  the  German 
standard  specification. 


Hydraulic  Modulus  =  %  sUica  +  %  ^^  +  %  ferric 


German  experience  has  shown  that  the  hydraulic   modulus 
should  never  be  less  than    1-7.     In   any  case,  the   hydraulic 


HYDRAULIC   MODULUS  65 

modulus  is  not  alone  sufficient  for  determining  the  proportions 
of  each  of  the  ingredients  used  in  making  Portland  cement. 

The  foregoing  limits  are  too  wide  in  some  respects,  for  it  is 
well  known  that  a  variation  of  2  per  cent,  in  the  proportion 
of  calcium  carbonate  in  the  slurry  or  mixture  before  it  enters 
the  kiln  will  make  all  the  difference  between  a  sound  and  an 
unsound  cement.  The  limits  mentioned,  and  shown  in  'Fig.  8, 
must,  therefore,  be  regarded  as  purely  approximate,  and  as 
of  academic  rather  than  empiric  value. 

In  .connection  with  this  diagram  it  is  interesting  to  note  that 
according  to  Asch's  theory  all  cements  should  lie  on  one  of  the 
four  vertical  .lines  marked  A.  The  height  above  the  base  line 
at  which  they  should  be  placed  depends  on  the  lime-content, 
which — according  to  the  same  theory — may  vary  within  limits 
similar  to  those  indicated  by  the  dotted  lines  on  the  chart, 
i.e.,  between  three  and  eighteen  molecules  of  lime. 

The  commercial  limits  of  composition  usually  recognised  are 
somewhat  larger  than  those  corresponding  to  Asch's  theory. 
This  is  due  to  the  impurities  in  the  materials  used  commercially. 

In  the  limiting  formulae  mentioned  on  pp.  62 — 64,  no  definite 
ratio  between  the  silica  and  alumina  is  stated,  yet  this  is  of 
great  importance,  as  cement  rich  in  silica  is  deemed  superior  to 
that  deficient  in  it.  Moreover,  cement  mixtures  low  in  silica 
when  burned  in  rotary  kilns  cause  the  formation  of  adherent 
rings  of  clinker  which  choke  the  kiln.  0.  Dormann  has  found 
that  the  best  cements  are  obtained  commercially  when  the 
limits  are  R^O^  :  Si0.2  ==  1  :  2-5  to  3-0  expressed  in  absolute 
weight,  or  1  molecule  A120.^  to  .each  4-25  to  5-1  molecules 
Si0.2,  and  a  study  of  numerous  published  analyses  of  German, 
American  and  British  cements  confirms  this  ratio,  very  few 
high-class  cements  with  an  A1203  :  Si02  ratio  lower  than  3-66 
(molecules)  having  been  found.  According  to  the  theory  of 
the  constitution  of  cements  given  on  pp.  55 — 59  the  Al.203  :  Si02 
ratio  should  be  definitely  4  or  5  (molecules). 

In  the  chart  shown  on  p.  63,  the  minimum  values  of  lime  are 
made  to  include  an  allowance  for  the  iron  oxide  by  assuming 
that  it  is  equivalent  to  that  of  the  alumina  present. 

The  lines  on  the  chart  show  the  relatively  narrow  range  of 
composition  permissible  in  cements,  but  it  must  not  be  for- 

c.  F 


66    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

gotten  that  they  are  not  defined  with  absolute  accuracy,  and 
future  investigations  into  the  constitution  of  cements  may 
result  in  an  even  narrower  limit  of  composition  being  imposed. 
The  assumptions  on  which  they  are  based  are  by  no  means 
free  from  objection  ;  thus,  it  is  not  by  any  means  correct  to 
add  together  the  percentages  of  alumina,  ferric  oxide  and  silica 
as  is  done  in  the  formula  for  the  hydraulic  modulus,  though, 
in  the  ordinary  manufacture  of  cement,  variations  in  the 
proportion  of  alumina  and  iron  are  sufficiently  small  to  permit 
such  formulae  to  be  conveniently  used  in  spite  of  their  theoretical 
inaccuracies.  For  the  same  reasons  the  lines  on  the  chart 
cannot  rightly  be  extended  very  far  beyond  the  positions 
shown,  as  it  has  been  found  that  cements  containing  a  ratio 
of  only  2-5  molecules  of  lime  to  1  molecule  of  silica  are  unsound. 

Chemical  Reactions  between  Silica  and  Lime. 

Owing  to  the  proportion  of  free  silica  in  the  raw  materials 
and  the  possible  decomposition  of  the  clay  into  a  mixture  of 
free  silica  and  alumina,  there  is  always  some  free  silica  present 
in  the  contents  of  the  cement  kiln  during  an  early  stage  of  the 
heating. 

As  well-made  Portland  cements  leave  only  an  insignificant 
proportion  of  insoluble  matter  when  treated  with  hydrochloric 
acid,  the  study  of  the  silicates  present  is  simplified  by  the 
exclusion  of  all  possible  silicates  which  are  insoluble  in  this 
acid.  For  this  reason,  synthetic  experiments  in  which  the 
substances  actually  present  in  the  raw  materials  used  (or 
supposed  to  be  produced  by  the  direct  action  of  heat  on  these 
materials)  are  heated  in  groups  of  twos  and  threes,  are  of 
the  utmost  importance.  The  products  obtained  from  pure 
materials  can  be  examined  with  accuracy  and  without  the 
disturbing  factors  present  when  less  pure  materials  are  used. 
Experimenting  in  this  manner,  J.  W.  Cobb  has  found  that  on 
heating  a  mixture  of  limestone  or  chalk  and  finely  powdered 
quartz,  a  reaction  occurs  at  800°  C.  (i.e.,  at  or  below  the  tem- 
perature at  which  free  lime  is  formed),  and  the  combination  of 
the  lime  and  silica  takes  place  with  increasing  rapidity  as  the 
temperature  is  raised,  a  soluble  silicate  being  formed.  At 


REACTIONS   BETWEEN  SILICA   AND   LIME       67 

temperatures  below  1250°  C.  no  fusion  is  observable,  though 
the  formation  of  the  soluble  silicate  CaOSi02  is  quite  definite. 
At  1400°  C.  (or  just  below  the  highest  temperature  in  a  rotary 
kiln)  the  formation  of  calcium  mono-silicate  is  practically 
complete.  Any  calcium  sulphate  present  is  also  decomposed 
by  the  silica,  calcium  mono-silicate  being  formed  at  tempera- 
tures above  1005°  C. 

Cobb  also  found  that  if  sufficient  silica  is  present,  the  pro- 
portion of  silica  and  lime  has  no  influence  on  the  result,  and 
that  the  compound  CaOSi02  is  invariably  formed.  When  the 
original  mixture  contained  the  materials  in  the  ratio  SCaO  -j- 
8i02  he  observed  the  formation  of  a  more  basic  silicate 
(2CaO  .  Si02)  at  first,  and  that  this  persists  in  the  presence  of 
sufficient  lime  ;  otherwise  CaO  .  Si02  is  formed.  Under  no 
circumstances  could  Cobb  produce  a  calcium  silicate  containing 
more  lime  than  2CaOSi0.2. 

The  apparent  impossibility  of  producing  a  compound 
corresponding  to  3CaOSi0.2  makes  it  very  difficult  to  accept 
the  largely-held  view  that  Portland  cements  contain  a  large 
proportion  of  this  substance.  As  already  indicated,  in  such  a 
view  the  alumina  present  in  cements  is  overlooked,  or  is,  at 
least,  regarded  as  forming  entirely  different  compounds. 

So  far  as  cements  are  concerned,  it  appears  improbable  that 
more  than  an  insignificant  proportion  of  any  calcium  silicate 
as  basic  as,  or  more  basic  than,  2CaOSi02  can  be  present,  and 
that  such  reactions  as  may  occur  between  free  lime  and  free 
silica  in  the  cement  must  finally  result  in  the  formation  of 
calcium  mono-silicate  (CaOSi02):  The  fact  discovered  by 
Boudouard,  that  CaOSi02  corresponds  to  the  most  fusible 
mixture  producible  from  lime  and  silica  alone,  implies  that 
this  substance,  if  present,  forms  part  of  the  inter-crystalline 
material  observable  in  all  Portland  cements,  and  as  it  is  devoid 
of  hydraulic  properties  it  is  of  no  value  in  cements. 

Calcium  ortho-silicate  2CaOSi02  is  present  in  cements  and 
slags  not  containing  sufficient  silica,  or  which  have  not  been 
heated  sufficiently  long  to  form  the  meta-silicate  CaOSi02. 
The  ortho-silicate  is  described  under  the  term  belite  (p.  51). 

Many  chemists  interested  in  cements  have  attached  great 
importance  to  the  investigations  of  Shepherd,  Day,  R-ankine. 


68    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

Wright  and  others  on  the  products  formed  by  fusing  binary 
mixtures  of  lime  and  silica,  or  lime  and  alumina,  and  ternary 
mixtures  of  lime,  alumina  and  silica,  and  allowing  the  fused 
mass  to  cool  slowly.  Under  such  conditions  various  crystalline 
substances  are  found  in  the  mass  produced,  the  most  important 
being — in  the  case  of  lime-silica  mixtures — a,  ft  and  y 
Ca2SiOi,  a  and  ft  CaSi03,  tridymite,  quartz  and  free  lime. 
In  burning  cements,  however,  complete  fusion  of  the  mass  is 
never  reached,  and  the  products  formed  in  a  mixture  which 
has  only  been  partially  fused  are  not  the  same  as  those  produced 
in  a  completely  fused  mass  of  the  same  original  composition. 
For  this  reason  the  application  of  the  work  of  the  above- 
mentioned  investigators  to  the  constitution  of  cement  clinker 
is  somewhat  misplaced.  Indeed,  no  less  an  authority  than 
C.  Doelter  (Zeitsch.  /.  Elektro  Chem.,  Bd.  17  (1911),  p.  795)  has 
reached  the  conclusion  that  "  the  application  of  the  phase  rule 
to  fused  mixtures  of  silicate  components  is  erroneous,"  and 
that  such  substances  "  must  not  be  treated  as  metallic  alloys, 
as  this  leads  to  erroneous  conclusions  which  only  increase  the 
difficulties  experienced  in  the  application  of  physical  chemistry 
to  mineralogical  problems."  Doelter  has  also  shown,  experi- 
mentally, that  silicate  mixtures  when  heated  above  their 
melting  point  produce  quite  different  substances  from  those 
which  are  formed  when  no  over-heating  has  occurred.  The 
study  of  such  completely  fused  mixtures  is,  therefore,  of  little 
value  in  studying  the  constitution  of  cements,  and  is  most 
likely,  as  Doelter  suggests,  to  lead  to  false  conclusions. 

The  size  of  the  grains  of  free  silica  has  an  important  influence 
on  the  reaction.  0.  Dormann  has  found  that  unless  the  free 
silica  is  fine  enough  not  to  leave  a  residue  of  more  than  20  per 
cent,  on  a  No.  180  sieve,  a  considerable  part  of  it  will  remain 
uncombined. 

Experience  shows  that,  within  certain  limits,  cements  with 
a  high  proportion  of  silica  are  stronger  than  others,  the  ultimate 
strength  increasing  approximately  with  the  percentage  of 
silica.  This  is  usually  attributed  to  the  larger  proportion  of 
tri-calcium  silicate  in  such  cements,  but,  in  view  of  the  doubts 
as  to  the  existence  of  tri-calcium  silicate  in  cements,  a  more 
probable  explanation  is  that,  within  the  limits  indicated,  the 


REACTIONS  BETWEEN  LIME  AND  ALUMINA       69 

ultimate  strength  really  depends  on  the  nature  as  well  as  on 
the  amount  of  alumino-silicate  formed. 

An  increase  in  the  percentage  of  silica  in  a  cement  will 
usually  be  accompanied  by  a  decrease  in  the  alumina  and  a 
reduction  in  the  speed  of  setting  of  the  cement.  This  is 
apparently  due  to  the  smaller  proportion  of  the  alumino- 
silicate,  which  is  the  essential  constituent  of  the  cement,  and 
to  the  correspondingly  larger  proportion  of  inert  matter 
present. 

Chemical  Eeactions  between  Lime  and  Alumina. 

It  is  by  no  means  certain  that  the  contents  of  a  cement  kiln 
ever  contain  an  appreciable  amount  of  free  alumina,  though 
it  may  possibly  be  produced  by  the  direct  action  of  heat  on 
clay.  Such  free  alumina  as  does  occur  may  enter  into  com- 
bination with  both  lime  and  silica,  forming  an  alumino-silicate, 
or  it  may  combine  solely  with  lime  to  form  one  of  two  calcium 
aluminates,  CaOAl203  or  CaO  2A120S,  both  of  which  are  soluble 
(with  decomposition)  in  hydrochloric  acid. 

The  reaction  between  lime  and  alumina  commences  (according 
to  J.  W.  Cobb)  at  850°  C.,  occurs  rapidly  at  1100°  C.,  and  is 
practically  complete  at  1300°  C.  According  to  the  proportion 
of  lime  and  alumina  either  CaOAl2O3  (which  is  soluble  in  acid), 
2CaOAl.2Os  (which  is  very  slowly  soluble)  or  an  insoluble 
calcium  aluminate  of  unknown  composition  is  produced. 
Cobb  has  been  unable  to  produce  a  calcium  aluminate  corre- 
sponding to  3  CaOAl.2Os  and  soluble  in  hydrochloric  acid.  The 
^CaOAl.203  obtained  from  molten  mixtures  at  1531°  C.  by 
Shepherd  and  others  is  not  produced  under  conditions  existing 
in  cement  kilns,  and,  as  already  explained  (p.  68),  it  is  erroneous 
to  suppose  that  a  substance  produced  when  a  completely  fused 
and  usually  overheated  mixture  is  allowed  to  cool  is  neces- 
sarily formed  when  the  same  raw  materials  are  heated  to  only 
partial  fusion.  Many  binary  silicates  and  aluminates  possessing 
hydraulic  properties  appear  to  exist,  but  in  all  probability  a 
large  number  of  these  are  incapable  of  production  by  the 
methods  at  present  employed  in  the  manufacture  of  cements. 
The  tri-calcium  aluminate  and  tri-calcium  silicate,  so  generally 


70    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

considered  to  be  essential  constituents  of  cements,  both  appear 
to  be  of  this  nature.  The  compound  5CaO3Al.203,  which  rnelts 
at  1386°  C.,  may  be  present  in  cements,  but  has  not  yet  been 
identified  therein. 

The  importance  of  such  calcium  aluminates  as  may  be  present 
depends  upon  the  proportions  in  which  they  occur  in  cements. 
Those  who  maintain  that  cements  consist  chiefly  of  a  mixture 
of  tri-calcium  silicate  and  tri-calcium  aluminate,  naturally 
consider  that  a  large  proportion  of  the  latter  aluminate  must 
be  present,  notwithstanding  the  fact  that  it  has  not  yet  been 
definitely  isolated  from  commercial  cements. 

The  di-calcium  compound  2CaOAl.,Os  prepared  by  Newberry 
is  a  quick-setting  hydraulite  with  constant  volume  and  good 
hardening  properties.  If  present  in  a  commercial  cement  it 
would  not  affect  the  quality  of  the  material,  though  it  might 
increase  the  rate  of  setting.  Hence,  it  is  generally  considered 
desirable  to  have  the  proportion  of  alumina  as  low  as  possible, 
so  as  to  secure  greater  ultimate  strength  and  slowness  of 
setting,  but  it  must  not  be  in  too  small  a  proportion,  or  the 
material  will  not  be  properly  burned  at  the  temperature 
ordinarily  used. 

In  other  words,  if  too  little  alumina  is  present  the  amount 
of  alumino-silicate  produced  will  be  insufficient  to  produce  a 
valuable  cement,  and  a  mixture  of  various  feeble  hydraulites 
with  only  a  little  true  cement  will  be  formed.  If  the  com- 
position of  cement  is  represented  as  x  molecules  CaO,  Si0.2  +  y 
molecules  2(7aO,  ALOB  it  is  difficult  to  conceive  how  the  presence 
of  alumina  can  reduce  the  temperature  at  which  the  cement 
clinker  is  formed,  because  a  mixture  of  di-calcium  aluminate 
and  tri-calcium  silicate,  when  heated,  does  not  form  a  clinker  at 
the  temperature  reached  in  cement  kilns.  The  behaviour  of 
a  completely  fused  mixture  which  has  been  heated  to  a  much 
higher  temperature  and  then  allowed  to  cool  does  not  have 
any  bearing  on  this  question,  and  the  almost  inevitable  con- 
clusion is  that  the  alumina  enters  into  combination  with  both 
the  lime  and  silica  and  produces  a  definite  alumino-silicate,  and 
not  a  solid  solution.  It  is  precisely  because  this  alumino- 
silicate  is  an  essential  constituent  of  the  cement  that  the 
percentage  of  alumina  in  the  raw  mixture  must  not  fall  below 


REACTIONS  BETWEEN  LIME,  ALUMINA  &  SILICA     71 

a  certain  limit,  the  exact  value  of  which  has  not  been  definitely 
ascertained,  though  it  appears  to  be  approximately  one- 
twentieth  of  the  weight  of  the  lime  in  the  cement.  If,  on  the 
contrary,  too  large  a  proportion  of  alumina  is  present,  the 
excess  above  that  needed  for  the  alumino-silicate  will  induce 
the  formation  of  calcium  aluminates,  which  produce  a  sticky 
and  adherent  clinker  of  less  strength  than  that  formed  from 
alumino-silicate  alone.  If  the  2CaOAl.20.3  theory  is  held,  it  is 
difficult  to  understand  why  an  increase  in  the  proportion  of 
alumina  above  one-third  of  the  weight  of  the  silica  present 
should  produce  a  weaker  cement  than  is  formed  when  the 
normal  proportion  of  alumina  is  present,  as  pure  di-calcium 
aluminate  forms  a  stronger  cement  than  pure  tri-calcium 
silicate.  The  chief  evidence  against  the  existence  of  calcium 
aluminates  in  appreciable  quantities  in  commercial  cements 
is  (a)  the  appearance  of  the  latter  under  the  microscope, 
(b)  the  fact  that  the  "  alite  "  crystals  contain  practically  all 
the  alumina  in  the  cement,  apparently  in  the  form  of  a  com- 
pound of  lime  alumina  and  silica,  and  (c)  the  absence  of  any 
colouration  when  Portland  cements,  are  heated  with  an  alcoholic 
solution  of  phenolphthalein,  whereas  both  the  synthetical 
calcium  aluminates  produce  a  strong  colouration  with  this 
indicator. 

Chemical  Reactions  between  Lime,  Alumina  and  Silica. 

J.  W.  Cobb  has  found  that  when  a  ternary  mixture  of  lime, 
alumina  and  silica,  corresponding  to  CaO  -f-  Al.20.3  -j-  lQSiO.2, 
is  heated  to  temperatures  ordinarily  reached  in  cement  manu- 
facture, the  reaction  begins  at  800°  C.  and  proceeds  slowly  up 
to  1300°  C.,  at  which  temperature  a  siliceous  mass  is  pro- 
duced which  is  unaffected  by  hydrochloric  acid.  A  similar 
mixture,  but  containing  a  much  larger  proportion  of  lime, 
porresponding  to  SCaO  -\-  Al.20.3  +  3Si0.2,  and  intended  to 
represent  Portland  cement  (though  it  has  at  least  3  per  cent, 
more  alumina  and  3  per  cent,  less  silica  than  the  average 
cement),  forms,  at  1250°  C.,  a  product  which  is  soluble  in 
hydrochloric  acid,  but  is  not  a  useful  cement  and  contains 
much  free  lime.  If  it  is  really  a  compound  it  has  a 


v  72    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

composition  corresponding  to  2-1  CaO,  0-4  ^4/.,03,  SiO.,.  J.  W. 
Cobb  inclines  to  the  view  that  the  insoluble  products  are  true 
ternary  compounds,  whilst  the  soluble  ones  are  mixtures  of 
binary  silicates  and  aluminates,  though  the  evidence  afforded 
by  his  experiments  is  by  no  means  conclusive  on  this 
point. 

A  further  idea  of  the  great  complexity  of  a  study  of  these 
compounds  may  be  gained  from  the  fact  that  in  the  product 
formed  by  heating  CaO  -f-  Al.fi  z  +  10  Si0.2  the  alumina  acts 
as  a  base  and  is  replaced  by  Na.fi,  but  in  the  analogous 
product  from  Na.fi  -\-  CaO  -f-  IQA1203,  the  alumina  acts 
precisely  like  silica,  i.e.,  as  an  acid,  the  last  named  compound 
bearing  the  most  striking  resemblance  in  many  of  its  properties 
to  that  from  CaO  -f  Alfi.^  WSiO.,. 

The  inter-reactions  of  other  substances  in  cements  are  extremely 
difficult  to  ascertain.  To  a  large  extent  the  felspathic, 
micaceous  and  other  complex  minerals  which  may  be  present, 
form,  either  alone  or  in  combination  with  the  lime  present,  a 
considerable  portion  of  the  amorphous  inter-crystalline  matter 
always  observable  when  a  polished  piece  of  cement  clinker  is 
etched  with  water  and  examined  under  the  microscope. 

The  influence  of  the  small  proportions  of  these  various 
silicates  cannot  be  ascertained  with  accuracy,  as  the  apparently 
complex  products  partake  of  the  nature  of  viscous  glass  or 
slags,  and  do  not  readily  crystallise  in  the  form  of  definite 
compounds.  Moreover,  attempts  to  cause  the  amorphous, 
glassy  matter  to  crystallise  invariably  effect  changes  in  its 
composition,  so  that  an  examination  of  the  crystals  so  formed 
is  of  little  real  value.  Cements  made  some  years  ago,  for  which 
stationary  kilns  were  used,  show  a  considerably  higher  pro- 
portion of  insoluble  matter  and  of  imperfectly  burned  material, 
most  of  which  is  inert  and  useless  in  the  cement.  The  use  of 
rotary  kilns,  working  at  higher  temperatures,  enables  a  superior 
cement  with  a  much  smaller  proportion  of  undesirable  and 
inert  matter  to  be  produced,  facilitates  the  formation  of  those 
compounds  which  have  a  hydraulic  value,  correspondingly 
reduces  the  proportion  of  uncombined  oxides  and  useless 
products,  and  increases  the  rate  at  which  the  cement  sets. 
Some  of  the  oxides  stated  in  analyses  as  being  present  in 


EFFECTS  OF  MAGNESIA  73 

cement  are  of  sufficient  interest  for  their  reactions  to  be 
described  briefly. 

Magnesia  is  only  present  in  small  quantities  in  the  better 
class  commercial  cements,  as  the  use  of  magnesic  materials 
is  prohibited  in  all  official  specifications  for  Portland  cement, 
but  larger  proportions  are  present  in  many  natural  cements. 
It  combines  with  alumina  and  silica  in  a  manner  analogous  to 
lime,  but  there  is  a  great  difference  of  opinion  as  to  the  value 
of  magnesic  cements.  Magnesia  needs  a  much  higher  kiln 
temperature  than  lime  in  order  that  it  may  combine  with 
silica  and  alumina  ;  hence  an  excess  of  it  in  a  cement  mixture 
is  undesirable,  as  it  will  largely  exist  in  an  uncombined  state 
unless  an  unusually  high  kiln  temperature  is  employed. 

Newberry  has  found  that  magnesia  up  to  20  per  cent, 
produces  a  satisfactory  cement  if  due  care  be  taken  in  mixing 
and  burning,  and  the  relatively  high  proportion  of  magnesia 
in  many  satisfactory  natural  cements  shows  that  magnesia 
is  not  in  itself  a  disadvantage,  providing  that  the  cement  is 
properly  burned.  Therein  lies  the  difficulty. 

Tests  on  cement  to  which  magnesia  has  been  added  just 
previous  to  gauging,  show  that  magnesia  causes  expansion, 
but  much  less  rapidly  than  in  the  case  of  free  lime,  and  the 
damage  effected  is  correspondingly*  less. 

Since  the  failure  of  a  French  bridge  and  a  portion  of  the 
Aberdeen  harbour  works,  it  has  been  customary  to  limit  the 
percentage  of  magnesia  compounds  to  a  maximum  of  3  per 
cent.  MgO. 

The  lime-magnesia  cements  differ  in  several  important 
respects  from  Portland  cements,  but  are  not,  at  present,  of 
sufficient  commercial  importance  to  warrant  further  description 
here. 

"  Alkalies  " — chiefly  potash  and  soda  silicates  and  alumino- 
silicates — are  only  present  to  a  small  extent,  as  a  large  pro- 
portion of  the  alkaline  oxides  are  volatilised  during  the  burning 
when  rotary  kilns  are  used,  so  that  when  present  only  in  small 
quantity  in  the  raw  mixture  they  are  unimportant.  According 
to  Hillebrand  the  alkalies  be'gin  to  volatilise  before  all  the 
sulphur  tri-oxide  has  been  expelled,  i.e.,  at  a  temperature  of 
about  1150°  C.  Being  very  fusible,  particularly  when  mixed 


74    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

with  lime,  alumina  and  silica,  they  form  a  part  of  the  amorphous 
glassy  material  of  an  inert  nature  observable  in  all  cements. 

Iron  compounds,  analogous  to  those  of  alumina,  appear  to 
be  formed  in  the  manufacture  of  cements,  but  their  nature  is 
by  no  means  well  known.  The  reducing  atmosphere  inside 
most  cement  kilns  causes  the  formation  of  ferrous  compounds,  in 
which  the  iron  oxide  behaves  precisely  like  a  base,  though 
calcium  ferrites  (xCaO,yFe.2Os)  appear  to  exist  in  small  quantities 
in  some  cements,  these  proportions  being  much  smaller  than 
is  generally  supposed.  Under  ordinary  conditions,  the  ferrites 
are  devoid  of  hydraulic  properties,  but  a  mixture  corresponding 
to  2CaO,Fe203  was  prepared  by  Newberry  and  produced  a 
black  slag  on  heating  ;  the  ground  slag  did  not  set  when  mixed 
with  water  in  the  cold,  but  on  exposure  to  a  temperature  of 
100°  C.  it  rapidly  formed  a  very  hard  mass  of  constant  volume. 

Newberry  also  found  that  a  cement  in  which  iron  oxide 
entirely  replaced  the  alumina  usually  present  sets  slowly  to 
a  sound  mass.  This  points  to  the  existence  of  ferro-silicates, 
rather  than  ferrites  in  cements.  These  calcium  ferro-silicates 
are  apparently  of  the  type  xCaO,yFe.>0.^z8iO.>,  but  very  little 
investigation  has  been  made  of  their  properties.  They  appear 
to  have  a  close  resemblance  to  the  corresponding  alumino- 
silicates  and  to  be  formed  in  a  similar  manner,  but  are  much 
less  hydraulitic.  "  The  fusibility  of  ferrous  alumino-silicates, 
their  strong  tendency  to  form  glasses  or  slags,  and  the  ease  with 
which  they  are  formed  at  the  temperatures  employed  in  cement 
burning,  make  it  highly  probable  that  these  compounds 
contain  the  greater  part  of  the  iron  present  in  the  cement. 
Their  nature  is  such,  however,  that  it  is  almost  impossible  to 
isolate  them.  Ferrous  alumino-silicates  prepared  synthetically 
have  a  close  resemblance  to  cements,  but  are,  on  account  of 
their  dark  colour  and  the  rapidity  with  which  they  set,  almost 
useless  for  commercial  cements. 

A  variety  of  Portland  cement,  in  which  iron  ore  replaces  the 
clay  ordinarily  used,  is  employed  in  considerable  quantities 
on  the  Continent  and  in  the  Panama  Canal  on  account  of  its 
resistance  to  sea  water.  In  this  case  the  iron  appears  to 
replace  the  alumina  in  ordinary  Portland  cement  and  to  form 
a  completely  analogous  compound.  It  has  not  yet  been 


EFFECTS  OF  ALKALIES  AND  IRON  75 

conclusively  shown  whether  such  iron  ore  cement  contains  a 
calcium  ferro-silicate  of  the  hexite-pentite  type  or  whether  it 
is  a  mixture  of  calcium  silicates  and  ferrites,  but  the  existence 
of  a  ferro-silicate  is  the  more  probable. 

There  is  a  custom  among  writers  on  cement  to  associate 
together  the  alumina  and  iron  expressed  in  the  form  of  ferric 
oxide  (Fe.20.3)  as  though  they  were  mutually  replaceable. 
Although  ferric  oxide  is,  to  some  extent,  capable  of  replacing 
alumina  in  a  hydraulite,  it  very  seldom  does  so  in  commercial 
cements,  because  the  reducing  action  of  the  kiln  gases  cause 
the  formation  of  ferrous  oxide,  which  is  instantaneously 
converted  into  ferrous  silicate  or  ferrous  alumino-silicate,  and 
then  takes  little  or  no  part  in  the  chemical  changes  which 
occur  in  the  kiln.  Very  few  Portland  cements  contain  an 
appreciable  proportion  of  ferric  oxide  (Fe.20s)  (notwithstanding 
the  fact  that  this  substance  is  reported  in  most  analyses)  ; 
the  greater  part  of  the  iron  is  in  the  ferrous  state,  and  exists 
in  combination  with  silica  and  alumina  as  a  glassy  mass  or 
slag.  It  would  be  more  correct,  in  most  cements,  to  regard 
the  iron  as  allied  to  the  lime  and  magnesia — as  a  fairly  powerful 
base — and  not,  as  is  so  often  the  custom,  to  associate  it  with 
the  alumina  and  silica  as  though  it  were  an  acid.  That  a 
small  proportion  of  the  iron  in  a  cement  is  present  in  the  form 
of  an  acid  radical  is  not  improbable,  but  the  greater  part  of  it 
is  certainly  not  in  this  form.  Hence,  in  empirical  formulae 
designed  to  express  the  limits  of  composition  of  a  cement 
mixture,  the  iron  oxide  (unless  more  than  10  per  cent,  is 
present)  is  preferably  omitted,  the  "  constant  "  in  the  formula 
being  altered  accordingly,  if  necessary. 

The  chief  objections  to  a  large  proportion  of  iron  compounds 
in  a  cement  are  the  very  dark  colour  of  the  cement,  the  reduced 
hydraulicity,  and  the  lower  fusing  point  of  the  materials  as  a 
whole,  with  a  consequent  change  in  the  conditions  under  which 
the  cement  is  burned,  and  an  increased  tendency  to  the  decom- 
position of  the  cementitious  compounds  desired  and  the 
formation  of  solid  solutions  of  simpler  silicates. 

Sulphates  and  sulphides — chiefly  calcium  sulphide  CaS, 
ferrous  sulphide  FeS,  and  calcium  sulphate  CaS04 — are  often 
present  in  small  proportions  in  cements  and  in  larger  proper- 


76    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

tions  in  slags.  They  are  removed  from  the  latter  by  quenching 
the  molten  slag  in  water  ;  the  sulphur  is  then  evolved  as 
sulphuretted  hydrogen,  H.>S.  Sulphides  occur  in  the  raw 
materials  used  for  cement-making,  but  the  greater  part  found 
in  cement  is  due  to  the  action  of  the  sulphur  in  the  fuel  used 
for  heating  the  kiln.  J.  W.  Cobb  has  found  that  a  mixture  of 
pure  chalk,  alumina  and  quartz,  in  proportions  corresponding 
to  Portland  cement,  absorbed  17  per  cent,  of  sulphur  tri-oxide 
during  a  twenty-four  hours'  heating  in  a  works  furnace, 
the  greater  part  of  this  being  absorbed  below  a  temperature 
of  800°  C.  The  reducing  atmosphere  present  in  most  cement 
kilns  prevents  the  absorption  of  so  high  a  proportion  of  sulphur 
tri-oxide  by  the  cement,  and  any  calcium  sulphate  produced 
during  the  earlier  stages  of  the  burning  would  be  again  dis- 
sociated at  a  temperature  above  1100°C.  Nevertheless,  it  is 
important  to  avoid  the  use  of  materials  and  fuel  containing 
sulphur  as  far  as  it  is  possible  to  do  so.  The  chief  danger 
arising  from  the  presence  of  sulphides  is  their  liability  to 
become  oxidised,  forming  sulphates  and,  at  the  same  time, 
expanding  and  tending  to  disintegrate  the  cement.  For  this 
reason,  a  higher  proportion  than  2f  per  cent,  of  sulphur  as 
tri-oxide  (SO%)  is  usually  prohibited  in  specifications.  The 
addition  of  slag  to  Portland  cement  usually  raises  the  pro- 
portion of  sulphur  trioxide  beyond  the  limit  just  mentioned 
as  permitted  by  the  British  Standard  Specification. 

In  order  to  regulate  the  setting  of  cement,  the  addition  of 
2  per  cent,  of  water  and  2  per  cent,  of  anhydrous  gypsum 
(CaSO^)  is  permitted  in  the  British  Standard  Specification.  The 
water  is  applied  in  the  form  of  steam  and  the  gypsum  in  the 
form  of  plaster  of  Paris.  Hence,  an  examination  of  most 
Portland  cements  will  show  a  somewhat  higher  proportion  of 
calcium  sulphate  than  is  derivable  from  the  raw  materials. 
This  additional  calcium  sulphate  is  not  included  in  the 
limit  of  2|"  per  cent,  previously  mentioned.  The  addition 
of  anhydrous  gypsum  has  a  greater  retarding  effect  on  the 
setting  than  the  equivalent  amount  of  plaster  of  Paris.  If, 
however,  the  cement  is  aerated,  the  gypsum  becomes  hydrated, 
and  its  retarding  effect  is  correspondingly  diminished. 

The  addition  of  gypsum  or  plaster  of  Paris,  or  the  presence 


EFFECTS  OF  SULPHUR  COMPOUNDS  77 

of  free  calcium  sulphate  in  a  cement,  may  be  shown  by  adding 
5  per  cent,  of  barium  carbonate  to  the  cement,  gauging  the 
mixture  and  testing  its  expansion  after  it  has  been  boiled  in 
water  (Le  Chatelier's  test).  The  barium  carbonate  decomposes 
the  calcium  sulphate  and  a  notable  increase  in  the  expansion 
of  the  cement  is  observable. 

Chemical  Changes  in  Manufacturing  Natural  Cements 
and  Hydraulic  Lime. 

In  the  so-called  Roman  and  natural  cements  made  from 
naturally  occurring  substances  (p.  13),  in  which  the  proportions 
of  lime  and  aluminosilicic  acid  are  such  as  enable  useful 
cements  to  be  made,  the  chemical  changes  which  occur  are 
precisely  the  same  as  those  described  in  the  foregoing  pages. 
The  final  product  is  very  inferior  on  account  of  (a)  the  lower 
temperature  in  the  kilns — whereby  the  reactions  between  the 
various  substances  do  not  proceed  so  completely — and  (b)  the 
absence  of  any  adjustment  of  the  composition  of  the  mixture, 
which  is  seldom  exactly  correct. 

Although  the  sintering  point  is  seldom  reached  in  burning 
natural  cements,  this  does  not  prove  that  certain  silicates  and 
aluminates  have  not  been  formed,  for,  as  already  noted, 
J.  W.  Cobb  has  ascertained  that  calcium  silicates  and 
aluminates  (and  also  alumino-silicates)  are  formed  at  tempera- 
tures far  below  their  melting  points. 

Unfortunately,  natural  cements  do  not  usually  form  a 
clinker  of  sufficient  rigidity  to  enable  them  to  be  polished, 
etched  and  examined  microscopically,  so  that  their  true 
composition  must  largely  be  argued  from  analogy.  The 
better  qualities,  which  yield  a  stronger  clinker,  bear  a  close 
resemblance  to  Portland  cement,  and  the  same  changes  may, 
therefore,  be  assumed  to  occur  as  take  place  in  the  manufacture 
of  the  latter  cement. 

For  the  same  reason,  the  nature  of  the  changes  which  occur 
in  the  burning  of  hydraulic  limes  cannot  be  stated  with 
accuracy.  These  limes  are  clearly  mixtures  of  free  lime  with 
some  form  of  pozzolana,  or  of  free  lime  and  a  kind  of  Portland 
cement. 


78    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

Such  natural  cements,,  therefore,  contain  free  chalk  as  well 
as  free  pozzolanic  material,  and  are  equivalent  to  a  mixture 
of  Portland  cement  containing  an  indefinite  quantity  of  raw 
materials,  and  of  the  products,  such  as  lime,  obtainable  from 
these  by  heating  at  a  relatively  low  temperature. 

Chemical  Changes  in  Manufacturing  Pozzolanas  and 
Slag  Cements. 

As  already  explained  (p.  16),  granulated  slags  differ  from 
other  raw  materials  used  in  the  manufacture  of  cement  in 
that  they  have  been  heated  to  fusion  and  then  cooled  rapidly 
by  quenching  with  water. 

Like  the  natural  pozzolanas,  the  granulated  slags  are  usually 
mixed  with  lime,  and  the  changes  which  occur  have  not  been 
studied  to  the  extent  as  those  occurring  in  the  burning  of 
Portland  cements.  That  any  form  of  combination  occurs 
between  the  lime  and  the  pozzolana  or  slag  when  the  materials 
are  in  the  dry  state  is  scarcely  to  be  expected.  In  all  proba- 
bility the  reaction  which  occurs  takes  place  as  soon  as  water 
is  added  to  the  cement.  The  nature  of  this  reaction  is  described 
more  fully  later  under  the  heading  "  Setting  and  Hardening." 

Some  basic  slags,  when  quenched  and  granulated,  contain 
sufficient  lime  to  produce  useful  cements,  though  much  less 
than  is  found  in  Portland  cement.  The  fact  that  the  produc- 
tion of  a  slag  involves  the  complete  fusion  of  the  material, 
accounts  for  the  reaction  being  more  complete,  so  that  a 
smaller  proportion  of  lime  is  sufficient.  Such  cements  are, 
however,  quite  different  in  constitution  from  Portland  cements, 
and  partake  largely  of  the  nature  of  unstable  glasses,  which 
are  readily  decomposed  by  water.  The  absence  of  all  crystal- 
lisation in  such  cements  makes  it  impossible  to  state  what 
compounds  are  contained  in  them  ;  they  may  be  solid  solutions 
of  various  calcium  silicates  and  aluminates  or  may  consist 
chiefly  of  a  calcium  alumino- silicate,  which  is  so  viscous  that 
it  retains  small  quantities  of  other  substances  in  solution. 
The  partial  crystallisation  which  occurs  when  such  slag  cements 
are  maintained  for  a  long  time  at  a  temperature  just  below  the 
melting  point  is  generally  considered  to  imply  the  existence 


PHYSICAL  CHANGES  DURING  MANUFACTURE      79 

of  several  binary  compounds  rather  than  one  impure  ternary 
one,  but  Doelter's  and  Dittler's  researches  point  to  the  opposite 
conclusion. 

The  Physical  Changes  during  Manufacture. 

The  physical  changes  which  occur  during  the  conversion  of 
the  raw  materials  into  cement  are  less  interesting  than  the 
chemical  changes  with  which  they  are  so  closely  associated. 
The  most  obvious  is  the  gradual  conversion  of  irregular  lumps 
of  clay  or  shale  and  chalk  or  limestone  into  a  slurry  in  the  wash 
mills,  or  a  powder  in  the  grinding  mills.  The  slurry  is  then 
dried  and  passed  to  the  kilns,  whilst  the  powdered  mixture  is 
either  made  into  briquettes  and  passed  into  a  stationary  kiln, 
or  it  is  taken  direct  to  a  rotary  kiln  as  described  in  Chapter  II. 

It  is  in  the  kiln  that  the  chief  physical  as  well  as  the  chief 
chemical  changes  occurring  during  manufacture  take  place,  the 
material  being  converted  from  a  friable  material,  or  loosely 
adherent  powder,  into  a  hard,  dark  grey  clinker  with  lighter, 
softer  portions  when  the  burning  has  been  less  complete.  A 
microscopical  investigation  of  this  clinker  will  show,  as  already 
explained  (p.  50),  that  it  consists  largely  of  a  preponderating 
crystalline  constituent  (alite  ?)  and  of  a  much  smaller  propor- 
tion of  a  glassy  or  slag-like  material,  the  nature  of  which  is 
not  easily  ascertained.  The  composition  and  texture  of  this 
glassy  material  vary  in  different  cements,  and  it  is  probably 
not  unreasonable  to  suppose  that  it  constitutes  the  residium 
of  all  the  ingredients  left  in  the  raw  materials  after  the  forma- 
tion of  the  alite  or  cement  proper. 

On  observing  a  sample  of  raw  mix  heated  gradually  to 
1450°  C.  by  means  of  a  Doelter's  microscope,  no  visible  change 
occurs  until  a  temperature  of  about  1375°  C.  is  reached,  slight 
fusion  then  occurs,  but  is  barely  appreciable  until  the  tempera- 
ture rises  to  1410°  C.  or  thereabouts,  when  a  sudden  formation 
of  (alite  ?)  crystals  takes  place  simultaneously  with  the  fusion 
of  the  remaining  constituents  of  the  material.  The  purer  the 
raw  materials,  i.e.,  the  nearer  they  approach  to  aluminosilicic 
acid  and  calcium  carbonate,  the  smaller  will  be  the  amount  of 
fusion  observable,  and  when  the  purest  possible  materials  are 


80    CHEMICAL  AND  PHYSICAL  CHANGES  IN  CEMENTS 

used  it  is  difficult  to  see  any  fusion  at  all  until  just  at  the 
moment  when  the  crystals  are  formed.  With  very  impure 
materials,  on  the  contrary,  or  where  the  proportion  of  lime  is 
too  large  and  the  clay  is  highly  siliceous,  the  production  of  a 
fused  material  in  relatively  large  quantities  may  be  observed 
at  a  temperature  of  about  1300°  C. 

Contrary  to  the  statements  of  some  writers  who  have  argued 
from  analogy  rather  than  from  direct  observation,  the  cement 
materials  do  not  fuse  gradually,  the  fused  portion  attacking  the 
remainder  and  bringing  more  and  more  of  the  material  into 
the  molten  state.  On  the  contrary,  the  combination  between 
the  lime  and  the  aluminosilicic  acid  occurs  without  fusion,  and 
as  soon  as  the  temperature  is  reached  at  which  fusion  does 
occur  the  compound  formed  (alite  ?)  begins  to  crystallise  with 
great  rapidity.  It  is  only  with  very  impure  materials  or  in 
very  abnormal  mixtures  that  a  large  amount  of  glassy  material 
is  produced. 

The  clinker  itself,  in  spite  of  its  hard  and  vitrified  appearance, 
is  an  extremely  porous  material  with  a  true  specific  gravity  of 
3  to  3*4  as  compared  with  2-5  in  the  case  of  the  raw  mix 
before  burning. 

The  slag  cements  are  usually  harder  and  less  porous,  whilst 
the  natural  cements,  pozzolanic  cements  and  hydraulic  limes 
are  quite  soft  and  more  free  from  noticeable  pores. 

Whilst  the  raw  materials  are  not  chemically  changed  in  con- 
tact with  water,  the  clinker  is  decomposed  in  a  manner  presently 
to  be  described,  and  whilst  dilute  acids  only  dissolve  the  cal- 
careous portion  of  the  raw  mix  and  leave  the  clayey  portion  as  an 
insoluble  residue,  the  same  acids  will  dissolve  clinker  completely. 

The  changes  in  the  optical  character  of  the  materials  are 
those  which  naturally  result  from  the  conversion  of  a  mixture 
of  amorphous  materials  into  a  vitrified  mass  composed  of 
crystals  with  glassy  inter-crystalline  matter. 

The  physical  properties  of  clinker  are  of  small  importance 
so  long  as  the  clinker  is  maintained  in  the  form  of  irregular 
lumps,  or  is  ground  to  the  fine  powder  in  which  commercial 
cements  are  sold  ready  for  use.  When  mixed  with  water, 
however,  the  changes  which  occur  are  so  striking  and  so 
important  that  they  must  be  described  in  a  separate  chapter. 


CHAPTER  IV 

THE     CHANGES     WHICH     OCCUR     IN     SETTING     AND     HARDENING 

THE  chemical  and  physical  changes  which  occur  when  a 
cement  is  mixed  with  water  are  both  numerous  and  important, 
but  are  so  complex  that  the  physics  and  chemistry  of  some  of 
them  cannot  readily  be  considered  separately.  They  are 
comprised  in  the  various  phenomena  which  are  known  collec- 
tively as  setting  and  hardening. 

When  a  cement  is  mixed  with  a  suitable  proportion  of  water,1 
it  is  converted  into  a  plastic  paste  which  may  readily  be 
moulded  into  any  simple  shape.  After  a  time,  the  mass  loses 
its  plasticity  and  becomes  more  rigid  and  the  water  disappears 
from  its  surface  ;  it  is  then  said  to  have  "  set."  After  a  much 
longer  period  it  becomes  intensely  hard  and  mechanically 
resistant.  The  period  between  the  first  addition  of  water  and 
this  loss  of  mobility  is  termed  the  initial  set.  With  a  quick- 
setting  cement  the  change  is  very  noticeable,  but  with  others 
it  is  so  indistinct  as  to  be  almost  unrecognisable.  In  some 
stages  this  division  into  two  separate  stages  is  strongly  marked, 
but  in  the  plasters  only  one  stage  is  observable.  It  is  important 
not  to  continue  the  gauging  after  the  initial  set  has  commenced 
or  the  interlocking  network  of  the  particles  will  be  destroyed, 
and  the  final  strength  of  the  cement  correspondingly  reduced. 
For  this  reason,  cements  should  be  gauged  in  small  quantities 
at  a  time  and  placed  in  position,  or  moulded,  before  an 
appreciable  amount  of  setting  occurs. 

The  proportion  of  water  affects  the  time  of  setting,  an  excess 
of  water  retarding  and  a  very  dry  gauging  accelerating  the 
setting.  An  increase  of  temperature  also  increases  the  speed 
of  setting  and  hardening  ;  hence,  cements  set  more  rapidly 
in  hot  climates  than  in  cooler  ones,  and  some  difficulty  is 

1  The  process  of  mixing  water  and  cement  to  form  a  paste  is  known  technically 
as  gauging. 

C.  G 


82        CHANGES  IN  SETTING  AND  HARDENING 

experienced  in  hot  weather  on  account  of  the  cement  setting 
much  more  quickly  than  usual. 

The  conversion  of  the  mixture  of  cement  and  water  into  a 
mass  of  a  hard,  stony  nature  may  be  due  to  one  or  more  of  the 
following  changes,  which  may  proceed  simultaneously  :— 

(a)  The  formation  of  a  crystalline   magma  from    a   super- 
saturated solution. 

(b)  The  desiccation  of  a  colloidal  substance  or  gel. 

(c)  The  reaction  of  various  substances  upon  each  other,  or 
with  water,  giving  rise  to  a  product  which  is  either  crystalline,  as 
in  (a),  or  colloidal,  and  is  later  desiccated  as  in  (b). 

Crystallisation  from  a  super-saturated  solution  is  charac- 
teristic of  the  setting  of  plaster  of  Paris  and  allied  substances, 
including  some  of  the  simpler  meta-silicates,  such  as  BaOSiO.>, 
CaOSi02,  etc.  The  first  stage,  or  "  setting,"  of  a  Portland 
cement  appears  to  be  of  this  nature,  crystallised  calcium 
hydrate  being  formed.  In  such  cases  the  substance  dissolves 
in  the  water  present,  more  material  entering  into  solution  than 
can  long  remain  in  that  state.  A  hydrate  is  then  formed  by 
combination  of  the  basic  portion  of  the  substance  with  the 
water,  and  as  this  hydrate  crystallises  out,  more  of  the 
anhydrous  material  is  dissolved,  the  process  being  continued, 
though  at  a  rapidly  diminishing  rate,  until  all  the  water  is 
combined  or  all  the  material  is  hydrated.  If  sufficient  water 
is  present  to  separate  the  crystals  from  each  other,  they  will 
not  form  a  cement,  though  the  chemical  changes  which  take 
place  are  identical  ;  the  setting  of  the  mass  under  the  conditions 
described  being  merely  a  question  of  the  spacial  relationship 
of  the  crystals. 

The  rate  at  which  such  setting  occurs  depends  on  the  purity 
of  the  solution  ;  the  presence  of  comparatively  small  quantities 
of  lime  and  other  colloidal  substances  in  solution  will  retard 
the  setting,  or  may  even  prevent  its  taking  place. 

Colloidal  substances  form  hard,  stone-like  products  in  an 
entirely  different  manner,  no  chemical  reaction  taking  place. 
When  a  substance  is  composed  of  extremely  minute  particles 
of  an  amorphous  character  it  possesses  a  number  of  peculiar 
properties  which  distinguish  it  from  the  same  substances  when 
in  a  crystalline  state.  Gelatin  or  glue  is  a  typical  colloid,  and 


CHANGES  IN  SETTING  AND  HARDENING        83 

its  colloidal  properties  are  easily  recognised,  but  calcium 
hydrate  (Ca0.2H.2),  clay,  silica,  alumina,  and  many  other 
substances  are,  under  certain  conditions,  of  the  same  nature. 

All  colloids  may  exist  in  three  forms,  (a)  one  in  which 
extremely  fine  particles  are  suspended  in  a  liquid  ;  this  mixture 
bears  a  close  resemblance  to  a  true  solution,  as  it  can  be 
filtered  without  removing  the  colloidal  substances  from  it  (such 
a  pseudo-solution  being  termed  a  sol)  ;  and  (b)  a  coagulated 
form  obtained  by  heating  the  sol  or  by  adding  acids  and  certain 
salts  whereby  the  substance  becomes  thick  and  slimy,  and  a 
plastic,  gelatinous  solid  or  gel  is  then  removable  from  it  by 
filtration  or  deposition.  Some  of  these  gels  and  sols  are 
convertible  into  each  other  an  indefinite  number  of  times,  but 
other  gels  (or  coagulated  form)  cannot  be  converted  into  the 
sol  form,  and  are  then  said  to  be  "  fixed  "  or  "  irreversible." 
(c)  A  third  form  consists  of  an  amorphous  one  corresponding 
to  the  dried  gel  ;  it  may  be  of  a  horny  or  a  granular  nature 
or  in  the  form  of  a  fine  powder.  Unless  "fixed"  it  may  be 
converted  into  a  sol  on  treatment  with  water,  though  some 
colloids  in  this  form  are  readily  hydrolysed  by  such  treatment. 

The  behaviour  of  a  colloid  is  most  readily  understood  by 
observing  that  of  gelatin  or  glue.  If  this  substance  is  placed 
in  water  it  gradually  swells  and  softens,  the  increase  in  volume 
being  very  large.  If  sufficient  water  is  present  the  gelatin  will 
appear  to  pass  into  solution  (sol  form),  otherwise  it  will  form 
an  exceedingly  soft  jelly  (gel  form).  If  this  jelly  is  removed 
from  the  water  and  allowed  to  dry,  it  will  gradually  shrink 
and  harden  until  a  strong,  horn-like  mass  is  produced.  If  the 
drying  is  carried  still  further  by  the  aid  of  heat,  the  dried 
material  may  be  rendered  irreversible,  and  will  not  soften  again 
when  placed  in  water.  Instead  of  drying  the  sol,  the  colloid 
may  be  coagulated  by  the  addition  of  various  chemicals, 
chiefly  of  an  acid  nature. 

Lime,  silica,  and  possibly  some  alumino-  or  other  silicates 
are  the  chief  colloids  found  in  cements  and  mortars  previous 
to  the  addition  of  water.  The  existence  of  free  lime  is 
natural  to  hydraulic  lime  and  mortar,  but  it  should  not  occur 
in  cement  which  has  been  carefully  made  and  well  burned. 
Free  lime  retards  the  setting  of  a  cement  or  mortar  containing 

G  2 


84        CHANGES  IN  SETTING  AND  HARDENING 

it,  as  on  treating  the  material  with  water  some  of  this  lime 
enters  into  solution  and  thereby  furnishes  one  of  the  products 
of  the  hydrolysis.  The  presence  of  such  a  reaction-product 
always  reduces  the  rate  of  a  chemical  reaction,  as  it  tends  to 
establish  an  equilibrium  earlier  than  would  occur  if  such  a 
product  were  entirely  produced  by  the  reaction  itself.  The 
free  lime  (if  any)  in  cement  and  mortar  must  not  be  confused 
with  that  produced  by  the  action  of  water  on  cement. 

The  colloidal  silica  present  in  some  cements  and  mortars 
before  they  are  mixed  with  water  is  never  large  in  amount, 
and  is  due  to  the  decomposition  of  adventitious  silicates  or 
aluminosilicic  acids  in  the  kiln.  The  behaviour  of  colloidal 
silica  obtained  in  this  manner  may  be  observed  under  the 
microscope,  as  described  later,  though  it  cannot,  with  this 
instrument,  be  distinguished  from  any  colloidal  alumino- 
silicic acid  which  may  be  produced  by  the  hydrolysis  of  an 
alumino-silicate  by  the  water  added  to  the  material. 

The  chief  changes  in  the  setting  and  hardening  of  cements, 
mortar,  or  other  hydraulites,  may  be  most  conveniently 
observed  in  connection  with  Portland  cement,  those  in  other 
cements  and  hydraulites  being  analogous,  but  less  easily  studied 
on  account  of  the  greater  irregularities  in  the  composition  of 
the  materials. 

As  already  explanied,  Portland  cement  consists  of  a  crystal- 
line constituent,  which  is  a  calcium  alumino-silicate,  together 
with  a  much  smaller  proportion  of  a  slag-like  or  glassy  mass, 
which  is  an  aggregate  of  the  various  "  impurities  "  contained 
in  the  raw  materials  used. 

When  the  cement  is  mixed  with  water,  allowed  to  harden, 
and  then  polished  and  examined  under  the  microscope,  it  will 
be  found  that  about  half  of  it  consists  oi  the  unaltered  grains 
of  cement  and  the  remainder  of  colloidal  or  gelatinous  material. 
The  latter  may  be  readily  distinguished  by  soaking  the  specimen 
in  an  aniline  dye,  such  as  methyline  blue  or  eosin  (red  ink), 
and  washing  out  the  unabsorbed  dye.  The  colloidal  matter 
retains  the  dye  so  persistently  that  it  cannot  be  removed  by 
simple  washing,  and  in  this  way  the  particles  of  colloid  may  be 
readily  distinguished  under  the  microscope  by  their  colour. 
The  proportion  of  unaltered  cement  depends  on  the  fineness 


CHANGES  IN  SETTING  AND  HARDENING        85 

to  which  the  cement  has  been  ground,  the  finest  particles  being 
the  most  readily  affected  by  the  water,  but  in  commercial 
cements  it  is  found  that  there  is  always  30  to  50  per  cent,  of 
unaltered  cement  in  the  fully  hardened  neat  cement.  By  the 
addition  of  sand  or  other  suitable  inert  material  the  particles 
of  cement  are  distributed  more  widely,  so  that  in  mixtures  of 
this  kind  a  more  complete  use  of  the  cement  is  made  than 
when  it  is  the  sole  constituent  of  the  material.  The  finer  the 
grains  of  cement,  the  greater  the  proportion  of  inert  material 
which  may  be  used,  and  the  more  complete  the  hydrolysis. 
It  is  important  to  observe  that  this  unaltered  cement  takes 
no  part  in  the  hardening  (unless  it  forms  a  nucleus  to  the 
colloid,  for  which  sand  is  equally  efficient).  Failure  to 
remember  this  has  led  to  several  fanciful  attempts  to  explain 
what  occurs  during  the  hardening  of  cements,  and  has  especially 
led  to  unnecessary  complex  theories  as  to  the  reason  that  a 
hardened  cement  will  set  a  second  time  if  re-ground  and  then 
again  gauged  with  water.  Careful  experiments  with  the  aid 
of  the  microscope  have  shown  that  so  long  as  unaltered  cement 
is  present  the  hardened  mass  may  be  re-ground  and  re-gauged 
an  indefinite  number  of  times,  the  limit  being  fixed  by  the 
amount  of  cement  which  is  hydrolysed  each  time  the  mass  is 
mixed  with  water.  Even  with  the  most  finely  ground  cements 
and  the  most  carefully  made  mixtures  of  these  with  sand,  the 
whole  of  the  cement  is  never  hydrolysed  the  first  time  the 
mixture  is  gauged  with  water. 

Those  portions  of  the  cement  which  have  been  altered  by 
contact  with  water  are  found,,  on  examination,  to  have  under- 
gone a  chemical  decomposition,  free  calcium  hydrate  (Ca0.2H.2) 
and  a  new  series  of  compounds  (hydro-silicates  and  hydro- 
alumino-silicates)  being  formed.  Water  acts  both  as  a  base  and 
an  acid  and  decomposes  the  cement,  liberating  lime  and  colloidal 
silica,  colloidal  aluminosilicic  acid  and,  possibly,  colloidal 
calcium  silicates.  The  lime  crystallises  as  hydrate  from  the 
solution,  and  the  remaining  substances  rapidly  assume  a 
gelatinous  form.  Careful  examination  under  a  microscope  will 
show  that  the  crystallisation  of  the  lime  occurs  somewhat 
rapidly,  especially  with  quick-setting  cements.  At  the  same 
time,  though  much  more  slowly,  the  other  minute  particles 


86         CHANGES  IN  SETTING  AND  HARDENING 

of  decomposed  cement  material  show  their  characteristically 
colloidal  nature  inasmuch  as  they  gradually  swell  and  lose 
their  well-defined  shape  (just  as  gelatin  does  in  water)  and 
produce  a  gelatinous  transparent  substance.  This  colloidal 
material  gradually  increases  in  density  and  hardness  as  the 
lime  set  free  by  the  hydrolysis  is  absorbed  by  the  gel,  and  this 
desiccation  of  the  colloid  continues  even  though  the  cement  is 
immersed  in  water.  That  the  effect  of  water  on  cement  is 
chemical  and  not  entirely  physical  is  shown  by  the  rise  in 
temperature  being  roughly  inversely  proportional  to  the  time 
of  setting  of  the  cement.  Slow-setting  cements  have  only  a 
trifling  rise  in  temperature.  For  a  long  time  no  satisfactory 
explanation  could  be  given  for  this  rise  in  temperature,  but 
it  is  now  recognised  that  it  occurs  at  the  moment  of  hydration 
of  the  lime  liberated  from  the  silicate  molecule  in  cements  which 
contain  no  free  lime.  In  hydraulic  limes  (in  which  free  lime 
as  well  as  "  cement  "  is  present)  a  development  of  heat  occurs 
at  first ;  this  is  due  to  the  hydration  of  the  lime.  Then,  after 
an  interval,  a  second  development  of  heat  is  observable 
corresponding  to  the  chief  one  occurring  in  Portland  and  other 
lime-free  cements,  and  produced  at  the  moment  when  the 
lime  is  first  separated  from  the  alumino-silicate  (alite  ?) 
molecule,  and  forms  free  calcium  hydrate  (Ca0.2H.>).  It  is  not 
improbable  that  the  water  first  attacks  the  glassy  constituents 
of  the  cement  and  that  the  rise  in  temperature  associated  with 
the  initial  set  is  due  to  this  chemical  reaction  on  the  amorphous 
material.  The  action  of  water  on  the  crystalline  alite  is 
probably  slower,  and  this  may  account  for  the  second  rise  in 
temperature  during  setting.  It  should  be  observed,  however, 
that  the  hardening  of  cements,  as  distinct  from  the  setting, 
does  not  admit  of  very  accurate  and  precise  study,  especially 
as  both  it  and  the  setting  occur,  to  some  extent, 
simultaneously. 

The  conflicting  views  as  to  the  constitution  of  Portland 
cements  have  their  counterpart  in  the  variety  of  substances 
which  are  supposed  to  occur  in  a  cement  which  has  been  mixed 
with  water  and  then  allowed  to  harden  completely.  Those 
chemists  who  maintain  that  cements  are  solid  solutions  of 
calcium  aluminate  and  silicate  aver  that  the  action  of  water 


CHANGES  IN  SETTING  AND  HARDENING         87 

on  cement  is  to  produce  a  mixture  of  colloidal  and  crystalline 
tri-calcium  aluminate  (3CaOAl.,0.^)9  colloidal  and  crystalline 
calcium  hydroxide  (CaO2H.2),  and  colloidal  calcium  silicate. 
The  existence  in  hardened  cements  of  free  calcium  hydroxide 
(Ca0.2H.,)  in  crystalline  form  was  discovered  by  Le  Chatelier, 
but  his  explanation  of  the  reaction  of  cement  with  water  cannot 
be  accepted,  as  it  refers  exclusively  to  the  behaviour  of  tri- 
calcium  silicate  (which  has  never  been  definitely  isolated  from 
cement),  and  omits  all  reference  to  the  alumina  present. 
Moreover,  the  analogous  barium  silicates  are  hydrolysed  and 
form  a  hard  mass  which  differs  from  Portland  cement  in 
being  crystalline  and  not  colloidal,  and  in  being  useless 
as  a  hydraulite  on  account  of  the  product  being  soluble 
in  water.  In  view  of  the  fact  that  the  chief  constituent 
of  cements  is  a  calcium  alumino-silicate,  it  appears  to  be 
far  more  probable  that  the  result  of  the  action  of  water 
will  be  to  form  such  compounds  as  are  shown  on 
p.  88,  in  which  a  portion  of  the  lime  has  become 
hydrolysed  and  has  been  liberated  as  Ca(OH)2.  The  total 
proportion  of  lirne  thus  set  free  need  not  be  large,  and  there 
are  reasons  for  supposing  that  only  a  proportion  of  the  lime 
attached  to  the  silicon  rings,  together  with  the  whole  of.  that 
attached  to  the  alumina  ring  (p.  55)  is  removed  from  the 
molecule. 

The  simplest  and  most  obvious  theory  explaining  the  setting 
and  hardening  of  cements  is  that  which  regards  these  pheno- 
mena as  hydration  processes  of  the  cement  molecule.  In 
fact,  it  follows  directly  from  Asch's  theory  of  the  chemical 
constitution  of  cements  (p.  55)  that  the  addition  or  substitu- 
tion of  a  large  number  of  hydroxyl  groups  is. possible,  and  W. 
and  D.  Asch,  Feichtinger,  and  others  have  also  found  experi- 
mentally that  substances  which  can  add  hydroxyl  (OH)  groups 
at  a  definite  rate  to  their  molecule  are  hydraulites.  The 
addition  of  water  must  be  progressive  and  must  not  occur  too 
rapidly.  The  experimental  evidence  suggests  that  any  sub- 
stance in  which  the  addition  of  the  OH  -groups  occurs  slowly 
at  first  and  progresses  at  a  suitable  rate  until,  finally,  a  very 
large  number  of  OH-groups  have  been  added,  will  form  a 
harder  and  denser  cement  than  if  a  smaller  quantity  of  water 


88        CHANGES  IN  SETTING  AND  HARDENING 


had  entered  into  the  reaction  or  if  the  reaction  had  occurred 
more  rapidly.1 

If  the  constitutional  formula  of  a  typical  cement  is  repre- 
sented in  accordance  with  Asch's  theory  as 

5CaO.  KO.  5CaO. 

4CaO. 


5CaO.  KO.  5CaO. 

FORMULA  A. 

it  can,  on   treatment  with  water,   form  numerous  hydrated 
compounds,2  such  as 


HO  .  Ca  .  O 
HO  .  Ca  .  O 


HO  .Ca  .  O\ 
HO  .Ca  .  O/ 

HO  .Ca  .  O\ 
HO  .Ca  .  O/ 


Si 


HO  .Ca  .0 
HO  .Ca  .0 


OK 


Al 


OK 


0  .  Ca  .  OH. 
0  .  Ca  .  OH. 


Si 


O  .  Ca  .  OH. 

\O  .  Ca  .  OH. 

/O  .  Ca  .  OH. 
.  Ca  .  OH. 


O  .  Ca  .  OH. 
O  .  Ca  .  OH. 


FORMULA  B. 


and  will  require  twenty-eight  molecules,  or  13-8  per  cent,  of 
water — a  figure  which  agrees  remarkably  closely  with  that  found 
experimentally  by  Zulkowski  in  a  "  perfect  "  cement  when  set. 
The  dissociation  of  the  alumino- silicate  molecule  marks  the 
commencement  of  the  conversion  of  the  mixture  of  cement 
and  water  from  a  plastic  paste  to  a  solid  mass.  This  is  known 
as  the  "  initial  set,"  the  time  required  for  it  to  occur  being 
termed  the  "  time  of  setting."  Cements  differ  greatly  in  this 

1  The    hardening    of    trasses   and   pozzolanas  with   lime   and    water    may   be 
explained  in  an  analogous  manner.      These  "calcined  clays  "  (p.  43)  first  react 
with  lime  and  then  add  hydroxyl  groups  to  the  calcium  salt  first  formed. 

2  For  other  typical  hydrated* compounds,  see  Asch's  Silicates  (Constable  &  Co., 
Ltd.,  London    1913), 


CHANGES  IN  SETTING  AND  HARDENING         89 

respect,  some  being  very  much  quicker  than  others.  The 
earlier  cements  (which  were  under-burned  in  parts  and  so 
contained  free  lime)  and  the  hydraulic  limes  set  slowly,  as  the 
free  lime  in  them  dissolves  in  the  water  and  hinders  the 
hydrolysis  of  the  crystals,  because  lime  is  one  of  the  products  of 
this  hydrolysis.  In  more  modern  Portland  cements  the  burning 
is  more  efficiently  carried  out,  no  free  lime  occurs  in  them, 
and  the  clinker  is  much  more  finely  ground,  with  the  result 
that  they  set  much  more  rapidly,  and  a  small  percentage  of 
plaster  or  gypsum  must  usually  bemadded  to  retard  the  setting 
or  it  would  take  place  before  the  mixing  of  the  cement  and  water 
is  complete. 

In  a  carefully  made  cement,  which  has  been  well-burned  in 
a  rotary  kiln,  the  effect  of  water  is  tri-fold  :— 

(a)  The    alumino-silicate   molecule    is    hydrolysed    and    is 
decomposed,    free    calcium    hydroxide    (Ca0.2H.2),    and    such 
substances   as    those    described    on    a    previous    page    being 
produced.     The  products  are  in  each  case  colloidal,  but  may, 
later,    become    crystalline.     The    colloidal    matter    gradually 
absorbs   the   free   calcium   hydroxide   and   hardens,    and   the 
network  of  crystalline  matter  formed  as  either  a  primary  or 
secondary  product  of  the  hydrolysis  adds  to  the  strength  of 
the  material. 

(b)  Any   calcium    ortho-silicate    present   in    the    cement    is 
hydrolysed,  calcium  hydroxide  and  a  hydrated  silicate  being 
produced— 

2CaO  .  Si02  +  (x  +  1)  H^O  =  CaO  .  Si02  .  xH^O  +  Ca  (OH)2 

(c)  With    alumino-silicates    the    corresponding   equation   is 
expressed  by  one  of  the  following  types  :— 

(1)  C 


(2)    Ca,5AkSilbOM  +  36#20  =  Cal9 

It  will,   however,   differ  slightly  according  to  the  particular 

calcium    alumino-silicate   present   and   to   the   extent   of   the' 

hydration.1 

1  That  the  proportion  of  lime  may  vary  within  wider  limits  than  is  generally 
recognised  is  implied  by  some  experiments  of  Fremy  (Compt.  rend  :  67,  1205),  and 
Zulkowsky  (Sonderabd.  1908),  in  which  as  much  as  14  per  cent,  of  lime  was 
removed  from  some  Portland  cements  without  destroying  their  power  to  harden 
when  mixed  with  water, 


90        CHANGES  IN  SETTING  AND  HARDENING 

Any  ferrates,  aluminates  and  ferro-silicates  will  be 
hydrolysed  in  a  similar  manner,  and  will  form  either  colloidal 
products  or  will  add  to  the  complexity  of  the  interlocking 
silicate  crystals.  It  was  at  one  time  thought  that  quick- 
setting  cements  owed  their  property  to  the  aluminates  they 
contained.  This  is  only  indirectly  the  case.  Cements  which 
are  rich  in  alumina  are  those  which  contain  most  alumino- 
silicate,  and  they  set  more  rapidly  because  of  this  and  of  the 
relative  absence  of  retarding  substances.  Some  of  the  Roman 
cements  set  rapidly  because  of  the  large  proportion  of  alumino- 
silicate  present.  Hence,  it  is  not  unreasonable  to  assume  that 
the  greater  part  of  the  strength  of  hardened  cements  is  due  to 
the  colloidal  matter  produced  by  the  action  of  the  water 
(including  the  absorbed  calcium  hydrate)  rather  than  to  the 
formation  of  any  network  of  crystals  which  may  occur.  A 
microscopical  examination  of  hardened  cements  which  have 
been  polished  confirms  this  view,  the  total  proportion  of 
crystalline  matter  being  small  compared  with  that  of  the 
colloid  or  .amorphous  matter  present. 

The  opinion  held  by  some  chemists  that  the  constituents  of 
hardened  cements  are  free  lime,  free  silica  and  free  alumina, 
and  that  the  "  initial  set  "  marks  the  hydration  of  the 
aluminates,  and  the  "  final  set  "  that  of  the  silicates,  does  not 
coincide  with  many  of  the  facts.  It  is  quite  true  that  these 
substances  all  occur  in  hardened  cements  and  mortars,  but 
they  are  in  proportions  which  are  far  too  small  to  correspond 
to  the  strength  of  the  material  as  a  whole.  To  a  very  large 
extent  the  existence  of  free  alumina  and  free  silica  in  large 
proportions  is  merely  academic  deduction  from  the  supposed 
existence  of  correspondingly  large  quantities  of  tri-calcium 
silicate  and  di-  or  tri-calcium  aluminate  in  cement  clinker, 
and  the  general  supposition  that  only  binary  compounds  of 
lime,  silica  and  alumina  exist  in  cements.  -The  proved  existence 
•  of  large  quantities  of  alumino-silicate,  and  especially  the 
recognition  of  the  chief  constituent  of  cement  as  a  definite 
alumino-silicate,  deprives  the  explanation  of  the  process  of 
setting  and  hardening  usually  found  in  text-books  of  much  of 
its  assumed  value.  The  products  of  the  action  of  water 
on  cements  are  not  free  silica  and  alumina,  but  complex 


CHANGES  IN  SETTING  AND  HARDENING        91 

aluminosilicic  acids,  some  of  which  contain  much  combined 
lime. 

It  was,  at  one  time,  thought  that  the  hardening  of  cements 
and  mortars  was  due  to  the  direct  combination  of  free  lime  with 
silica  and  the  formation  of  calcium  silicate.  No  combination 
can  occur  between  sand  and  lime  at  ordinary  temperatures, 
but  active  aluminosilicic  acid  (such  as  occurs  in  pozzolanic 
materials)  will  absorb  free  lime  and  form  a  hard  colloidal 
mass  which  increases  in  hardness  as  the  water  in  it  is  removed. 
The  great  hardness  of  many  ancient  mortars  is  due  to  the  use 
of  ground  tiles  and  other  pozzolanic  material  (capable  of 
providing  active  colloidal  silica,  alumina  and  aluminosilicic 
complexes)  in  addition  to  the  sand  ordinarily  used. 

The  action  of  carbon  dioxide  (in  the  atmosphere)  on  cements 
and  mortars  is  always  of  a  secondary  character  so  far  as 
hardening  is  concerned.  The  primary  action  of  carbon  dioxide 
is  on  the  free  calcium  hydroxide  present — 

Ca(OH}*  +  CO*  =  CaC03  +  H20. 

The  lime  thus  carbonated  is  rendered  insoluble  and  useless 
as  far  as  any  further  reaction  is  concerned,  but  the  calcium 
carbonate  rapidly  assumes  a  crystalline  form,  and  the  inter- 
laced network  of  crystals  slightly  increases  the  strength  of  the 
whole  mass.  The  greater  part  of  the  hardness  and  strength  is, 
however,  due  to  the  colloidal  matter  present,  as  already 
explained,  the  action  of  carbon  dioxide  being  almost  entirely 
confined  to  the  surface  of  the  material  and  rarely  penetrating 
into  the  interior  even  after  2,000  years,  as  may  be  seen  by 
examining  the  walls  of  ancient  buildings. 

Some  carbon  dioxide  (about  5  per  cent,  under  favourable 
conditions)  is  absorbed  by  most  cements  when  they  are  stored 
or  "  matured."  The  amount  absorbed  was,  at  one  time, 
thought  to  correspond  to  the  free  lime  present  in  the  cement, 
but  more  recent  investigations  have  shown  that  well-made 
Portland  cements  contain  no  free  lime,  and  indicate  that  the 
action  of  carbon  dioxide — which  requires  the  simultaneous 
presence  of  water — is  due  to  a  slight  decomposition  of  the 
cement. 

As  the  finest  particles  will  react  the  most  rapidly,  a  definite 


92         CHANGES  IN  SETTING  AND  HARDENING 

degree  of  fineness  is  necessary  to  the  production  of  a  satisfac- 
tory hydrated  product,  but  too  fine  a  material  will  react  so 
rapidly  as  to  form  a  feeble  cement,  unless  its  action  is 
retarded  by  the  addition  of  a  suitable  agent. 

RETARDATION  OF  SETTING. 

The  retardation  of  setting  becomes  increasingly  important 
as  the  manufacture  of  cements  is  improved.  Portland  cements 
burned  in  stationary  kilns  are  comparatively  slow-setting,  but 
those  burned  in  rotary  kilns  set  too  rapidly  for  convenient 
and  satisfactory  working.  The  rate  of  setting  of  a  cement 
may  be  decreased  in  two  ways  :  (a)  the  cement  may  be  exposed 
to  the  air  (i.e.,  aerated),  or  (b)  a  small  proportion  of  a  retarding 
agent  may  be  added,  some  form  of  calcium  sulphate  being 
usually  employed.  Other  salts  may  be  used  instead  ;  thus, 
P.  Rohland  found  the  time  of  setting  of  cement  was  shortened 
by  adding  calcium  chloride,  aluminium  chloride,  potassium 
sulphide,  sodium  carbonate,  potassium  carbonate,  aluminium 
sulphate  and  alum.  The  time  is  lengthened  by  potassium 
dichromate,  boric  acid,  borax,  sodium  sulphate,  potassium 
sulphate  and  calcium  sulphate.  Exposure  to  air  is  really 
equivalent  to  exposure  to  moisture,  and  steam  is,  therefore, 
used  as  a  retarding  agent  with  good  effect.  This  is  due  to  the 
hydration  of  a  small  portion  of  the  cement  and  the  consequent 
liberation  of  a  little  free  calcium  hydrate.  When  the  clinker 
is  treated  with  steam  so  that  about  1  per  cent,  water  is  absorbed, 
the  proportion  of  calcium  sulphate  required  is  reduced  to 
about  one-half  that  otherwise  needed. 

The  precise  nature  of  the  action  of  calcium  sulphate  on 
cement  is  not  fully  understood.  As  only  2  to  3  per  cent,  is 
required,  its  action  may  be  largely  catalytic.  Various  other 
salts  produce  a  retardation  when  used  in  small  quantities  and 
an  acceleration  when  larger  proportions  are  used,  though  the 
results  of  investigations  on  this  subject  are  far  from  conclusive 
and  are,  in  some  instances,  mutually  incompatible. 

E.  Candlot  has  also  stated  that  a  double  salt  corresponding 
to  the  formula  3CaOAl.2O^CaSO^H,0  is  formed  by  the  inter- 
action of  calcium  sulphate  and  cement,  and  that  this  compound 
is  insoluble  in  water,  and  so  converts  any  aluminate  into  a  form 


RETARDATION  OF  SETTING  93 

in  which  it  takes  no  part  in  the  setting.  This  explanation  is 
widely  accepted,  but  can  hardly  be  said  to  meet  the  facts. 
Candlot  has  endeavoured  to  remove  some  of  the  objections  to 
this  theory  by  postulating  that  a  certain  quantity  of  free  lime 
is  necessary  in  order  that  the  gypsum  may  have  its  effect. 
It  is  true  that  a  partially  hydrated  cement  is  retarded  by  a 
much  smaller  quantity  of  calcium  sulphate  than  a  cement 
which  has  not  been  treated  with  steam  or  moisture,  but  this 
does  not  necessarily  prove  Candlot 's  theory.  A  more  probable 
explanation  is  that  the  calcium  sulphate  dissolves  at  such  a 
rate  that  its  saturated  solution  prevents  the  hydrolysis  of 
the  cement  by  enabling  only  a  very  small  proportion  of  lime 
to  be  liberated  and  dissolved  at  a  time.  Whichever  theory 
be  adopted,  it  is  a  curious  fact  that  the  highly  soluble  calcium 
chloride  has  an  even  stronger  retarding  action  than  calcium 
sulphate,  but  magnesium  chloride  accelerates  the  setting. 
This  implies  that  it  is  the  acid  portion  of  the  material  ($04, 
Cl,  etc.)  which  is  the  active  agent,  and  that  some  combination 
of  these  ions  and  the  cement  molecule  occurs.  The  destructive 
action  of  sulphate  solutions,  and  particularly  of  sea-water  on 
cement  immersed  in  them,  confirms  the  opinion  that  some 
reaction  occurs  between  the  $04-ion  and  the  cement. 

In  a  hydrated  or  hardened  Portland  cement  such  as  is 
represented  by  Formula  B  (p.  88),  the  two  OK  groups  are 
readily  replaceable  by  monovalent  acid  radicals  such  as 
S02.OH,  so  that  when  such  a  cement  comes  in  contact  with  a 
solution  of  calcium  sulphate  there  is  a  great  probability  that 
an  alumino-silicate  will  be  formed  in  which  this  replacement 
has  occurred.  Such  alumino  silicates  are  well  known,  and 
include  many  sodalites  and  ultramarines.  As  the  replace- 
ment is  accompanied  by  a  change  of  volume,  it  will,  if  at  all 
extensive,  tend  to  cause  the  cracking  and  ultimate  disintegra- 
tion of  the  cement.  Arguing  in  this  manner,  W.  &  D.  Asch 
have  suggested  that  the  only  way  to  prevent  adventitious 
sulphates  (including  those  in  sea  water)  from  affecting  con- 
crete structures  is  to  avoid  all  cements  of  the  type  a,  p.  56,  in 
which  these  readily  replaceable  groups  attached  to  the 
aluminium  hexite  are  present,  and  to  use  exclusively  cements 
in  which  they  do  not  exist,  e.g.,  cements  of  the  type  6,  p.  56.. 


94        CHANGES  IN  SETTING  AND  HARDENING 

The  retarding  action  of  calcium  sulphate  is  diminished  by 
storing  the  cement.  This  has  been  explained  as  being  due  to 
the  gradual  carbonation  of  the  lime  set  free  by  the  sulphate 
and  moisture,  but  it  is  equally  probable  that  a  combination 
between  the  sulphate  and  the  cement  occurs  on  long  storage 
or  on  exposure  to  the  atmosphere. 

Calcium  sulphate  is  usually  added  to  cement  clinker  in  the 
form  of  gypsum,  the  materials  being  ground  together.  Finely 
powdered  gypsum  may  also  be  added  to  the  ground  cement, 
but  it  is  preferable  to  use  plaster  of  Paris,  which  is  obtainable 
in  a  much  finer  state  of  powder  than  is  gypsum.  The  amount 
of  either  gypsum  or  plaster  needed  depends  on  the  proportion 
of  CaSO^  present  and  on  the  fineness  of  its  particles  ;  not  on 
any  imaginary  difference  in  the  chemical  activity  of  these  two 
substances. 

Experiments  as  to  the  effect  of  the  addition  of  2  to  3  per 
cent,  of  calcium  sulphate  to  various  quick-setting  cements 
show  curiously  irregular  results,  and  indicate  that  the  action 
of  this  substance  is  by  no  means  so  simple  as  is  sometimes 
supposed.  Some  of  the  discrepancies  may  be  due  to  lack  of 
uniformity  in  the  cement  itself  or  to  irregular  admixture  of 
the  retarding  agent.  Whatever  the  cause,  the  disadvantages 
of  adding  such  retarders  should  not  be  overlooked,  and  the 
amount  used  should  be  kept  as  small  as  possible. 

The  whole  subject  of  the  action  of  retarding  agents  and 
the  changes  which  take  place  when  the  cements  are  stored  is 
worthy  of  further  investigation.  The  inherent  difficulties  of 
the  subject  are,  however,  very  serious. 

Aeration,  or  exposure  of  cement  to  the  atmosphere,  reduces 
the  rate  of  setting  and  so  acts  as  a  retarder.  It  also  hydrates 
any  finely  divided  particles  of  free  lime  which  may  be  present, 
and  so  reduces  their  tendency  to  expand  or  "  blow  "  at  a 
later  stage  in  the  use  of  the  cement.  The  changes  which  occur 
in  aeration  are  similar  to  those  effected  by  water,  but  the 
small  proportion  of  water  present  in  the  atmosphere  makes 
the  changes  much  slower.  The  carbon  dioxide  in  the  air 
converts  any  hydrated  lime  into  microscopic  crystals  of 
calcium  carbonate  and  so  renders  them  inert.  Excessive 
exposure  to  air  reduces  the  value  of  the  cement  by  effecting 


AERATION  OF  CEMENT  95 

the    hydrolysis  and    carbonation  of    the  finest  and  therefore 
most  valuable  particles. 

The  earlier  cements  were  improved  by  aeration,  which 
hydrated  any  quicklime  present  and  so  reduced  it  to  a  fine 
powder,  but  well-made  modern  cements  are  not  improved  by 
this  treatment.  Nevertheless,  it  is  a  wise  precaution,  when 
testing  cements  which  yield  unsatisfactory  results,  to  expose 
them  to  the  air  for  three  or  four  days  and  then  test  again  ; 
they  will  then,  in  many  cases,  yield  satisfactory  results. 

NORMAL   RATES    OF   SETTING. 

For  information  on  the  rates  at  which  setting  and  hardening 
occur,  the  reader  should  refer  to  p.  109,  et  seq. 


CHAPTER  V. 

TESTING    THE    PROPERTIES    OF     CEMENTS. 

THE  primary  object  of  all  cement-testing  is  to  determine 
whether  the  material  is  satisfactory  in  two  important  particu- 
lars— strength  and  soundness.  Other'  tests — for  fineness, 
specific  gravity,  the  time  of  setting  and  chemical  analysis — 
are  only  of  value  as  additional  information  on  the  general 
suitability  of  the  material. 

Of  all  materials  which  are  regularly  tested  in  chemical  and 
physical  laboratories  there  are  none  in  which  the  tests  are  more 
dependent  on  the  judgment  and  skill  of  the  operator  than 
cements,  and  even  in  the  best  equipped  testing  stations  it  is 
impossible  to  avoid  a  large  personal  equation.  For  this 
reason  tests  carried  out  by  amateurs  are  usually  of  little  value 
until  the  necessary  manipulative  skill  has  been  attained. 

It  is  of  the  greatest  importance  in  studying  results  obtained 
in  testing  cements  that  the  precise  manner  in  which  the  tests 
have  been  performed  should  be  known.  Thus,  differences  in 
the  proportion  of  water  used  in  the  gauging  will  cause  great 
discrepancies,  and  may  lead  to  erroneous  conclusions.  Where 
the  tests  are  supposed  to  have  been  made  in  accordance  with 
a  standard  specification,  care  should  be  taken  to  see  that  the 
instructions  have  been  fully  carried  out.  Even  then,  there  is 
still  room  for  widely  differing  results,  because  two  testers, 
working  quite  independently  on  the  same  cement,  may  have 
different  ideas  as  to  the  precise  meaning  of  "  normal  con- 
sistency "  as  applied  to  gauged  cements. 

Users  of  cement  are  frequently,  though  quite  unconsciously, 
very  unfair  in  their  manner  of  judging  the  value  of  different 
cements.  Some  of  them  prefer  to  make  what  they  call  a 
"  practical  test,"  that  is  to  say,  they  observe  the  behaviour  of 
cements  from  different  sources  when  in  use  for  actual  construc- 
tional work.  This,  at  first  sight,  appears  to  be  the  best  of  all 


TESTING  CEMENTS  97 

tests,  but  it  overlooks  the  enormous  influence  of  the  "  personal 
equation."  A  little  carelessness  or  lack  of  skill  on  the  part  of 
the  workmen  employed  will  result  in  the  production  of  inferior 
work,  and  may  lead  to  the  condemnation  of  a  cement  of 
exceptionally  good  quality.  There  are  many  ways  in  which 
good  cements  may  be  spoiled  by  faulty  manipulation  and  by 
the  use  of  unsuitable  aggregates,  and  to  obtain  uniformly 
satisfactory  results  needs  the  unceasing  exercise  of  vigilance, 
care  and  skill  on  the  part  of  everyone  concerned. 

Other  users  base  their  judgment  on  unimportant  matters 
and  conclude  that  a  cement  which  hardens  more  slowly  than 
another  must  be  inferior,  or  they  condemn  a  cement  which, 
after  twenty-eight  days,  does  not  show  a  large  increase  in 
strength  when  its  original  strength  is  greatly  above  the  normal. 
Only  as  users  are  prepared  to  study  cements  scientifically 
will  the  true  value  be  appreciated  and  the  best  results 
obtained. 

The  improvements  effected  in  the  manufacture  of  cements, 
and  particularly  of  Portland  cement,  are  due,  in  large  measure, 
to  the  imposition  of  standard  specifications  for  cements  in  all 
the  more  important  countries.  These  standards  differ  some- 
what from  each  other,  partly  on  account  of  the  differences  in 
the  climatic  conditions,  and  partly  because  of  minor  variations 
in  the  manner  in  which  some  of  the  tests  are  carried  out. 
These  variations  are  rapidly  disappearing  in  consequence  of 
the  work  of  the  International  Association  for  Testing  Materials. 
In  the  following  pages  only  brief  notes  are  given  as  to  the 
various  properties  with  the  limits  set  in  the  specifications,  and 
as  these  limits  are  varied  from  time  to  time  and  the  details  of 
the  official  tests  are  occasionally  altered,  it  is  desirable  that 
the  specification  in  force  at  any  particular  time  should  be 
consulted.  The  British  Standard  Specification  may  be  obtained 
from  the  Engineering  Standards  Committee,  28,  Victoria 
Street,  Westminster,  London,  S.W.,  price  5s.  3d.,  post  free. 
This  relates  exclusively  to  Portland  cement,  no  official  standard 
for  other  cements  having  yet  been  prepared. 

In  drawing  up  this  specification  for  use  by  engineers,  the  chief 
aim  has  been  to  select  properties  which  will  indicate  a  lack 
of  soundness  in  the  cement  when  in  use,  together  with  such 

0.  H 


98        TESTING  THE  PROPERTIES  OF  CEMENTS 

other  properties  as  will  ensure  the  closest  similarity  between 
various  batches  of  cement  or  cements  made  by  different  firms. 
It  is  recognised  that  great  variations  exist  in  the  composition 
of  the  raw  materials  used,  and  the  specification  is  therefore 
arranged  so  as  to  secure  a  maximum  of  uniformity  with  a 
minimum  of  disturbance  to  existing  manufacturers. 

Not  only  is  care  and  skill  needed  in  carrying  out  the  tests 
themselves,  but  the  manner  in  which  the  samples  are 
taken  from  the  bulk  must  be  such  as  will  yield  "  fair " 
samples. 

The  procedure  recommended  in  the  British  Standard  Specifi- 
cation is  generally  adopted  in  Great  Britain,  viz.  : — 

"  Each  sample  for  testing  shall  consist  of  approximately  equal 
proportions  selected  from  twelve  different  positions  in  the 
heap  or  heaps  when  the  cement  is  loose,  or  from  twelve  different 
bags,  barrels,  or  other  packages,  when  the  cement  is  not  loose, 
or  where  there  is  a  less  number  than  twelve  different  bags, 
barrels,  or  other  packages,  then  from  each  bag,  barrel,  or  other 
package.  Every  care  shall  be  taken  in  the  selection,  so  that 
a  fair  average  sample  may  be  taken. 

"  When  more  than  250  tons  of  cement  is  to  be  sampled  at 
one  time  separate  samples  shall  be  taken  from  each  250  tons  or 
part  thereof." 

CHEMICAL    COMPOSITION. 

The  chemical  composition  of  Portland  cements  has  been 
limited  by  the  definition  of  Portland  cement  as  "  the  product 
resulting  from  the  burning  of  an  intimate  admixture  of 
calcareous  and  argillaceous  materials  as  principal  ingredients, 
which  burning  is  carried  to  the  point  of  incipient  fusion,  the 
clinker  produced  being  ground  to  a  fine  powder."  In  the  most 
recent  British  Standard  Specification  the  composition  is  still 

CaO 
further    regulated    by    the    ratio    r^ — , — TTTT  (expressed  in 

OtC/g   -\-  A-lvU^ 

molecular  equivalents),  being  limited  between  2-85  as  a  maxi- 
mum and  2-0  as  a  minimum.  This  limited  range  of  com- 
position is  intended  to  exclude  cements  containing  granulated 
blast-furnace  slag.  (See  p.  62.) 


COMPOSITION   OF  PORTLAND   CEMENT          99 

The  maximum  amount  of  the  following  constituents  is  also 
fixed  at  the  figures  stated  below  :— 

Per  cent. 

Insoluble  residue  .  .  .  .  1-5 
Magnesia  .....  3-0 
Loss  on  ignition l  .  .  .  .2-0 
Sulphur  trioxide  (S0.3)  .  .  .  2-75 

APPARENT  DENSITY. 

The  apparent  density  of  a  cement  is  the  ratio  of  the  weight 
of  a  given  volume  of  cement  to  its  volume,  but  the  expression 
is  of  small  value  unless  the  method  of  filling  the  measuring 
vessel  with  cement  is  known.  If,  for  example,  the  powder  is 
introduced  into  a  glass  flask,  the  latter  being  tapped  gently  from 
time  to  time,  a  very  different  figure  will  be  obtained  for  the 
apparent  density  than  if  no  tapping  or  shaking  is  permitted. 
The  usual  practice  is  to  employ  a  funnel  provided  with  a 
stopper  at  its  lower  end.  A  sufficient  quantity  of  cement  is 
placed  in  this  funnel  and  is  allowed  to  run  into  a  cylindrical 
measure  placed  beneath  until  the  latter  is  filled  to  overflowing. 
The  excess  of  cement  is  removed  from  the  receiver  by  drawing 
a  straight-edge  across  the  top  of  the  latter  so  as  to  leave  it 
exactly  full.  The  receiver  with  its  contents  is  then  weighed. 

The  receiver  may  be  of  any  convenient  capacity.  In  former 
days  it  held  exactly  one  bushel  (=  8  gallons),  but  at  the 
present  time  it  is  more  frequently  1  litre.  From  the  larger 
measure  the  "  weight  per  bushel  "  is  easily  ascertained  ;  from 
the  latter  the  "  litre-weight  "  is  equally  easy.  It  is  desirable 
to  check  the  capacity  of  the  receiver  by  weighing  or  measuring 
the  water  it  holds  when  filled  exactly  to  the  brim. 

The  object  of  ascertaining  the  apparent  density  is  to  gain 
some  idea  as  to  whether  the  cement  has  been  under-burned, 
as  the  more  complete  the  burning  the  greater  will  be  the  weight 
per  bushel  or  litre-weight.  This  test  is  complicated  by  the 
fact  that  the  fineness  and  age  of  the  cement  both  reduce  the 
apparent  density,  so  that  it  is  of  little  use  in  comparing  cements 

1  Unless  it  can  be  shown  that  the  cement  has  been  ground  for  more  than  four 
weeks. 

H2 


100      TESTING  THE  PROPERTIES  OF  CEMENTS 

from  different  sources,  but  is  of  value  to  the  manufacturer 
who,  by  its  means,  is  able  to  check  the  correctness  or  other- 
wise of  the  calcination  of  the  cement.  With  the  increasing 
use  of  rotary  kilns  and  the  consequent  reduction  in  the 
proportion  of  under-fired  clinker,  the  necessity  for  determining 
the  apparent  density  is  gradually  disappearing. 

The  usefulness  of  this  ratio  is  also  limited  by  the  fact  that 
the  weight  of  ten  bushels,  or  litres,  or  any  other  volume,  does 
not  correspond  to  that  of  a  smaller  number.  The  size  and  shape 
of  the  measuring  vessel  have  a  great  influence  on  the  relative 
positions  of  the  particles.  For  the  same  reason  cement  cannot 
be  measured  instead  of  weighed  by  dividing  the  weight  required 
by  the  apparent  density.  The  volume  thus  calculated  differs 
sufficiently  from  the  true  volume  to  cause  an  appreciable 
difference  in  the  strength  of  the  mortar  for  which  the  cement 
is  used.  Where  the  cement  all  comes  from  the  same  works, 
however,  and  the  true  relation  between  weights  and  measures 
on  a  large  scale  has  been  found,  the  apparent  density  may  be 
used  with  more  accuracy. 

The  weight  per  bushel  of  a  good  Portland  cement  will  vary 
between  95  and  115  Ibs.,  and  the  litre- weight  from  1,010  to 
1,400  grammes,  but,  as  already  mentioned,  these  figures  are 
reduced  by  grinding  the  cement  more  finely,  and  by  storing 
it  for  some  time,  when  the  particles  become  partially  hydrated 
and  carbonated. 

The  specific  gravity  test  has  for  several  years  replaced  the 
weight-per-bushel  and  litre-weight  tests  in  this  country,  as  the 
latter  are  unreliable  with  very  finely  ground  cements. 

SPECIFIC  GRAVITY. 

The  specific  gravity  of  any  substance  is  the  ratio  of  the 
weight  of  that  substance  to  the  weight  of  an  equal  volume  of 
water.  In  the  case  of  fine  powders,  such  as  cements,  the 
specific  gravity  differs  from  the  apparent  density  because  the 
former  relates  to  the  volume- weight  of  the  individual  particles, 
whilst  the  latter  relates  to  the  volume-weight  of  the  mass,  and 
includes  the  space  between  the  particles  which  is  occupied  by 
air.  Hence,  to  determine  the  specific  gravity  it  is  necessary 


ASCERTAINING  SPECIFIC  GRAVITY  101 

to  measure  the  total  volume  of  all  the  particles,  each  one  being 
considered  separately.  To  do  this,  there  is  introduced  into 
a  flask  or  bottle  with  a  very  narrow  neck1  sufficient  paraffin, 
turpentine,  or  other  convenient  fluid,  which  is  without  action 
on  the  cement,  until  it  reaches  the  prearranged  mark  on  the 
neck  of  the  vessel.  The  vessel  with  its  contents  is  then  weighed 
accurately,  and  the  weight  noted.  The  vessel  is  then  partly 
emptied  and  a  definite  weight  of  cement  introduced  through 
a  funnel.  The  vessel  is  then  refilled  to  the  mark  with  the 
fluid,  tapped  gently  to  loosen  air-bubbles,  and  its  weight 
again  noted.  If 

F  =  the  weight  (in  grammes)  of  the  empty  vessel, 
T  =  the  weight  of  the  vessel  when  filled  with  fluid, 
W  —  the  weight  of  the  vessel  when  containing  cement  and 

also  fluid, 

C  =  the  weight  of  cement  introduced  into  the  vessel, 
S  =  the  specific  gravity  of  the  fluid  used, 

OS 

then  the  specific  gravity  of  cement  =  ^—    ^       ^  • 

It  is  by  no  means  easy  to  determine  the  specific  gravity  of 
cement,  as  the  air  adheres  very  closely  to  the  particles,  and 
some  of  the  latter  are  liable  to  be  carried  to  the  top  of  the  fluid, 
and  even  to  rise  above  it.  Only  by  very  careful  tapping  in 
a  horizontal  direction  can  the  cement  be  kept  in  its  place  below 
the  surface  of  the  liquid.  On  no  account  must  the  vessel  be 
shaken  vertically,  of  an  accurate  determination  will  be  rendered 
impossible  on  account  of  the  cement  which  will  lodge  on  the 
upper  part  of  the  neck  of  the  vessel.  Several  patterns  are  in 
use,  but  one  of  the  most  convenient  is  that  devised  by  W.  H. 
Stanger  and  B.  Blount,  which  is  a  modification  of  one  used  by 
Le  Ch atelier.  It  consists  of  a  flattened  flask  with  a  narrow 
neck  graduated  in  one-tenths  of  a  c.c.  The  capacity  of  the 
flask  to  the  lowest  graduation  is  64  c.c.,  and  this  is  marked  14, 
the  remainder  of  the  larger  graduations  being  marked  succes- 
sively 15,  16,  17  and  18,  so  that  the  capacity  of  the  flask  to 
the  top  graduation  is  exactly  68  c.c.  This  flask  is  carefully 

1  The  use  of  an  ordinary  stoppered  specific  gravity  bottle  is  inadvisable  on 
account  of  the  floating  of  the  finest  cement  particles. 


102      TESTING  THE  PROPERTIES  OF  CEMENTS 

dried  and  exactly  50  c.c.  of  paraffin  is  introduced  into  it  by 
means  of  a  pipette,  great  care  being  taken  not  to  wet  the 
graduated  portion  of  the  neck  of  the  flask.  Then  exactly 
50  grammes  of  cement  is  added  through  a  funnel,  and  the 
flask  is  gently  tapped  to  remove  air-bubbles.  The  level  of 
the  liquid  is  then  read  on  the  graduations.  This  number 
divided  into  50  will  give  the  specific  gravity  of  the  cement. 
Thus  if  the  liquid  reached  to  15-8  the  specific  gravity  of  the 
cement  is  50  -r-  15-8  =  3-16.  The  cumbersome  calculation 
which  is  necessary  when  a  specially  designed  vessel  is  not  used 
may,  by  this  means,  be  avoided.  The  cement  should  be 
introduced  after  the  paraffin,  as  otherwise  it  is  difficult  to  get 
a  sharp  reading. 

Unlike  the  apparent  density  (p.  99),  the  specific  gravity  is 
not  affected  by  the  fineness  of  the  cement,  but  the  specific 
gravity  diminishes  as  the  age  of  the  cement  increases,  in 
consequence  of  the  absorption  and  chemical  combination  of 
moisture  and  carbon  dioxide  from  the  atmosphere,  whereby 
partial  hydration  and  carbonation  of  the  cement  are  effected. 

The  chief  uses  of  the  specific  gravity  are :  (a)  To  distinguish 
Portland  cement  from  natural  cement  and  particularly  from 
that  form  of  the  latter  known  as  Belgian  cement  (p.  31). 
Genuine  Portland  cement  has  a  specific  gravity  between  3'0 
and  3'4,  whilst  natural  cement  has  a  specific  gravity  below 
3-0. 

(b)  The  specific  gravity  is  also  used  as  a  test  of  the  value  of 
a   cement  in  relation  to   the    extent    to    which    the    clinker 
has  been  burned,  but  the  difference  between  the  specific  gravity 
of  under-burned  and  normal  clinker  is  too  slight  to  be  relied 
upon.     It   not  infrequently   happens   that   a  cement   of   low 
specific  gravity  is  of  greater  strength  than  one  of  high  specific 
gravity,  so  that  no  important  conclusions  should  be  based  on 
this  test. 

(c)  In  testing  for  adulterants  in  cement  the  specific  gravity 
is  of   little    value    unless   the    added  material  is  present  in  a 
very  large  proportion.      The  specific  gravity  of  Kentish    rag 
— a  sandy  limestone  at  one  time  much  used  as  an  adulterant 
— is  2-9  ;    that  of  basic  slag  is  still  closer  to  that  of  cement, 
so  that  the  differences  are,  for  most  purposes,  insignificant. 


ASCERTAINING  SPECIFIC  GRAVITY  103 

In  short,  although  Portland  cement  has  a  specific  gravity  of 
3-00  to  3-40,  which  is  higher  than  that  of  other  cements,  and  this 
enables  a  somewhat  denser  mortar  to  be  produced,  a  low 
specific  gravity  does  not  necessarily  indicate  an  inferior  cement, 
as  the  absorption  of  water  and  carbonic  acid  from  the 
atmosphere  will  cause  a  considerable  reduction  of  the  specific 
gravity  and  yet  may  not  lower  the  value  of  the  cement. 

The  British  Standard  Specification  imposes  a  minimum 
specific  gravity  of  3*15  for  fresh  Portland  cement  and  3-10 
for  cement  which  has  been  ground  more  than  four  weeks 
previous  to  testing.  The  lower  limit  for  older  cement  is  to 
allow  for  the  change  in  specific  gravity  which  occurs  when 
cement  is  hydrated  and  part  of  the  lime  present  is  converted 
into  calcium  carbonate  on  exposure  or  storage.  The  effect  of 
the  age  of  the  cement  on  the  specific  gravity  may  largely  be 
eliminated  by  heating  the  cement  to  a  temperature  of  1,000°  C. 
(bright  red  heat)  for  a  short  time.  This  treatment  drives  off 
the  water  and  carbonic  acid  which  have  been  absorbed,  but 
the  cement  is  not  really  re-converted  into  a  properly  burned 
cement,  as  the  hydration  effected  by  the  moisture  in  the 
atmosphere  to  which  the  cement  was  originally  exposed 
causes  a  decomposition  of  the  cement  which  reheating  at  the 
temperature  mentioned  does  not  restore.  For  most  purposes, 
however,  the  error  introduced  into  the  specific  gravity  figure 
is  so  small  that  it  may  be  neglected. 

FINENESS. 

The  size  of  the  particles  of  cement  is  a  matter  of  the  greatest 
importance,  as  the  reactions  between  the  cement  and  water — 
which  give  the  material  its  chief  value — depend  upon  it. 
The  test  for  fineness  is  also  highly  important,  because  fine 
cement  has  a  much  greater  binding  power,  and  much  larger 
proportions  of  aggregate  may  therefore  be  used  than  with  a 
coarser  cement,  or,  conversely,  the  strength  of  the  material 
will  be  much  greater  for  the  usual  proportions  of  aggregate 
if  a  finely  ground  cement  is  used. 

It  is  sometimes  stated  that  the  "  flour  "  or  finest  particles 
contain  the  whole  of  the  cementitious  material.  This  is  not 
strictly  correct,  though  sufficiently  so  for  many  purposes. 


104      TESTING  THE  PROPERTIES  OF  CEMENTS 

Careful  tests  of  the  coarser  particles  will  show  that  they  are 
cementitious,  but  that  they  are  less  rapidly  attacked  by  the 
water  used  in  gauging.  On  grinding  the  coarser  particles  to 
flour  they  have  the  same  cementitious  value  as  the  fine  particles 
from  which  they  have  been  separated.  The  difference  in  their 
behaviour  is  entirely  due  to  the  relative  amount  of  surface 
exposed  and  not  to  any  other  chemical  or  physical  difference. 
With  coarse  particles  the  relative  surface  area  is  much  less  than 
with  finer  ones,  so  that  the  water  can  only  react  to  a  much 
smaller  extent,  and  the  final  product  is  much  weaker  than 
would  be  the  case  if  finer  cement  were  used.  In  addition  to 
this,  the  finer  particles  can  be  distributed  over  a  much  larger 
quantity  of  aggregate  when  the  cement  is  made  into  mortar 
or  concrete,  so  that  the  finer  a  cement  is  ground  the  less  will  be 
the  proportion  of  cement  needed  to  produce  a  mortar  of  given 
strength.  Commercially  this  is  very  important,  as  the  cement 
is  by  far  the  most  costly  ingredient. 

There  is,  however,  another  reason  why  cement  should  be 
finely  ground,  namely,  its  much  greater  freedom  from 
"  blowing  "  and  cracking,  particularly  if  it  be  underburned  or 
overlimed.  This  has  clearly  been  shown  by  D.  B.  Butler,  who 
found  that  a  number  of  cements,  in  the  state  in  which  they 
were  received  from  the  manufacturer,  formed  pats  which  were 
badly  blown  under  trying  conditions,  yet  the  same  cements 
when  re-ground,  so  as  to  pass  completely  through  a  No.  180 
sieve,  gave  perfectly  sound  pats. 

According  to  W.  Michaelis,  only  those  particles  are  of 
value  which  pass  through  a  305  X  305  sieve.  Hence  the  old 
methods  of  grinding  gave  only  50  per  cent.,  but  the  best 
modern  ones  yield  not  less  than  70  to  75  per  cent,  of  the  only 
valuable  constituent  of  cement. 

In  the  face  of  these  results,  it  is  not  surprising  that  the 
compressive  strength  of  mixtures  of  fine  cement  with  three 
parts  of  normal  sand  exceeds  that  of  normal  cement  and 
normal  sand  by  over  1,400  Ibs.  per  square  inch.  A  still  finer 
raw  material  will  increase  the  strength  still  more. 

Better  quality,  higher  commercial  value,  with  moderate 
increase  in  the  cost  of  production,  are  the  chief  advantages 
resulting  from  fine  grinding. 


THE  FINENESS  OF  CEMENT 


105 


The  only  drawback  to  fine  grinding  is  the  increased  rate  at 
which  the  cement  sets  ;  this  is  usually  overcome  by  treating 
the  hot  clinker  with  steam  and  adding  about  2  per  cent,  of 
calcium  sulphate  or  other  retarder  (p.  92). 

To  produce  a  cement  of  very  great  fineness  is  necessarily 
costly,  and  there  is,  therefore,  a  tendency  not  to  grind  more 
finely  than  the  user  considers  necessary.  Some  years  ago  a 
residue  of  10  per  cent,  on  a  No.  50  sieve  and  20  per  cent,  on  a 
No.  76  sieve  was  con- 
sidered to  be  good 
grinding  and  is  now 
customary  for  some 
of  the  cheaper 
cements.  For  Port- 
land cement  of  good 
quality,  however,  the 
leading  makers  now 
grind  so  that  there  is 
less  than  3  per  cent, 
on  a  No.  76  sieve, 
and  the  tendency  is 
to  demand  increas- 
ingly fine  grinding. 

In  order  to  obtain 
so  fine  a  product 
r  a  pi  dly -  driven 
grinding  machines 
must  be  excluded,  as 
they  are  not  suitable 
for  very  fine  grinding. 

Tube-mills  and  millstones  can  grind  very  fine,  but  for  a 
residue  of,  say,  10  per  cent,  on  a  175  X  175  sieve  their  output 
is  so  small  that  they  cannot  be  used  commercially  ;  if  a  still 
finer  product  is  required,  e.g.,  2  per  cent,  on  a  175  X  175  sieve, 
the  output  is  insignificant.  There  can,  on  the  contrary,  be 
no  question  as  to  the  ability  of  ball-mills  to  grind  to  any 
desired  degree  of  fineness,  whenever  a  suitable  separating  or 
sifting  device  is  available.  Such  an  arrangement  must  not 
operate  in  the  rough  and  ready  manner  of  a  sieve,  but  must 


FIG.  9.— Air  Separator  (Gebr.  Pfeiffer). 


106      TESTING  THE  PROPERTIES  OF  CEMENTS 

only  remove  the  very  finest  particles  and  must  return  the 
remainder  to  the  mill  so  that  it  may  be  crushed  still  finer. 
Such  an  apparatus  appears  to  exist  in  the  "  Selector  " — a 
form  of  air-separator  (Fig.  9) — which  can  easily  produce  a 
cement  with  as  little  as  2  per  cent,  residue  on  a  175  X  175  sieve 
with  an  output  of  80  per  cent,  of  that  obtained  when  cement  of 
normal  fineness  is  ground.  The  product  of  such  a  device, 
when  tested,  gave  Michaelis  the  following  results  :— 


Test  1. 

Test  2. 

Per  cent. 

Per  cent. 

Residue  on 

a  75  X  75  sieve 

0                 0 

Between  a 

75  X     75  and  167  X  167  sieve    . 

2                 0-5 

;> 

167  X  167  and  305  x  305  sieve    . 

28 

24-5 

?> 

305  X  305  and  610  X  610  sieve    . 

28 

25 

Through  a 

610  X  610   

42 

50 

Modern  Portland  cements  contain  about  55  per  cent,  of 
flour  separable  by  a  current  of  air,  the  remainder  being  in  the 
form  of  a  very  fine  grit.  In  cement  plants  using  air-separators 
there  is  a  somewhat  larger  proportion  of  flour  than  when 
screens  are  used. 

The  fineness  of  a  cement  is  ascertained  by  sifting  the  material 
through  carefully  standardised  sieves.  In  all  the  chief  coun- 
tries of  the  world  two  kinds  of  standard  sieves  are  used  ;  the 
coarser  has  seventy-six  holes  per  linear  inch  or  900  per  sq.  cm., 
and  the  finer  has  180  holes  per  linear  inch  or  4,900  per  sq.  cm. 
Finer  sieves  are  also  used  in  testing  laboratories  for  special 
investigations,  though  it  is  almost  impossible  to  use  sieves  with 
more  than  250  holes  per  linear  inch  on  account  of  the  clogging 
which  ensues. 

In  selecting  a  sieve  it  is  of  the  greatest  importance  that  all 
the  holes  should  be  exactly  the  same  size,  as  otherwise  the 
particles  which  pass  through  the  sieve  will  be  so  irregular  as 
to  make  the  results  useless.  The  recognised  standard  is  to 
make  the  holes  twice  as  wide  as  the  diameter  of  the  wire. 
For  the  No.  76  sieve  the  British  standard  size  of  wire  is  0-0044 
inch  diameter,  and  for  the  No.  180  sieve  it  is  0-0022  inch. 


THE  FINENESS  OF  CEMENT  107 

The  sieves  and  gauze  ordinarily  sold  by  wire  merchants  are 
quite  useless,  the  wires  being  too  irregularly  spaced,  and  some 
of  the  gauze  is  twilled  instead  of  being  evenly  woven.  Messrs. 
Greening  &  Sons,  Limited,  of  Warrington,  are  regarded  as  the 
semi-official  makers  of  suitable  gauze.  The  manner  in  which 
the  gauze  is  attached  to  the  frame  is  important.  It  must  not 
be  stretched  or  strained  over  a  circular  frame,  as  this  alters  the 
shape  of  the  holes  and  destroys  the  value  of  the  gauze.  The 
proper  method  is  to  use  a  square  frame  of  wood  or  metal  about 
three  inches  deep,  and  to  attach  the  gauze  to  this  by  means  of 
a  thin  supplementary  frame  or  slips  of  wood  screwed  to  the 
former  one.  It  is  very  convenient  to  make  the  sieves  fit  into 
each  other,  the  upper,  coarser  one  being  provided  with  a  close- 
fitting  lid,  and  the  lowest  with  a  box  to  receive  the  finest 
material.  This  arrangement  enables  the  sifting  to  be  carried 
out  without  creating  any  dust,  and  is  more  rapid  than  when 
each  sieve  is  used  separately. 

To  drive  the  fine  particles  through  the  sieve  it  is  necessary 
to  shake  it  continuously,  an  operation  which  requires  a  certain 
amount  of  skill.  In  the  British  Standard  Specification  it  is 
directed  that  100  grammes  or  4  ounces  of  cement  is  to  be 
continuously  sifted  for  a  period  of  fifteen  minutes  on  each  sieve. 
The  area  of  the  sieves  is  not  stated,  but  the  mesh  is  Nos.  76 
and  180,  respectively.  The  residue  on  the  coarser  sieve  must 
not  exceed  3  per  cent.,  and  that  on  the  finer  sieve  1-8  per  cent. 
Mechanical  contrivances  for  shaking  the  sieves  are  frequently 
used,  but  do  not  produce  such  satisfactory  results  as  hand 
shaking,  the  vibration  being  much  sharper  when  the  sieve  is 
mechanically  shaken.  The  use  of  a  fine  sieve  is  so  tedious  that 
several  attempts  have  been  made  to  separate  the  finest  particles 
by  other  methods.  The  use  of  a  washing  or  elutriating  appa- 
ratus with  paraffin  as  the  levigating  fluid  is  seldom  practical, 
as  it  involves  the  use  of  enormous  volumes  of  paraffin  which 
cannot  be  readily  purified  for  repeated  use.  Attempts  to 
separate  the  finer  particles  by  a  process  of  sedimentation  in 
paraffin  have  also  proved  unsatisfactory.  Results  of  reasonable 
reliability  have  been  obtained  by  Gary  and  Lindner,  and 
independently  by  Cushmann  and  Hubbard,  who  passed  a 
current  of  air  through  a  series  of  three  vessels  of  different  sizes. 


108      TESTING  THE  PEOPERTIES  OF  CEMENTS 

The  diameter  of  each  vessel  is  arranged  to  correspond  to  a 
convenient  speed  of  air  and  to  give  a  product  of  which  the 
particles  are  within  very  narrow  limits  of  size.  The  air  is 
admitted  at  a  pressure  of  100  mm.  water  column  to  the  bottom 
of  the  first  vessel,  and  passing  through  it  is  then  carried  to  the 
bottom  of  the  second  vessel,  and  so  on  throughout  the  whole 
apparatus.  A  convenient  quantity  of  cement  (usually  20 
grammes)  is  introduced  into  the  first  vessel  and  is  separated 
by  the  air-current,  the  particles  being  carried  along  in  propor- 
tion to  their  fineness.  The  largest  particles  remain  in  the 
first  vessel,  the  smallest  pass  through  the  apparatus  into  a 
collecting  vessel,  and  particles  of  intermediate  fineness  are 
left  in  the  second  and  third  vessels  respectively.  When 
carefully  used,  this  apparatus,  which  is  known  as  a  flouro- 
meter,"  gives  fairly  concordant  results,  though  these  are  always 
subject  to  a  loss  of  about  5  per  cent,  of  the  original  material. 
The  flourometer  is  of  insignificant  value  in  distinguishing 
cements  and  adulterants  ;  its  chief  value  lies  in  showing  the 
thoroughness  or  otherwise  of  the  grinding. 

The  author  has  obtained  highly  satisfactory  results  with  a 
modification  of  a  form  of  centrifugal  apparatus  patented  by 
W.  J.  Gee.  In  this  appliance  paraffin  of  a  definite  density  is 
mixed  with  the  cement  to  form  a  thin  slip  or  cream  which  is 
then  run  into  the  top  of  a  rapidly  rotating  cylinder.  Clear 
paraffin  passes  out  at  the  bottom  of  the  apparatus  and  on 
opening  the  latter  the  cement  is  found  to  be  graded  accurately, 
the  finest  particles  being  at  the  lower  end.  The  separation  is 
remarkably  sharp  and  repeated  tests  have  confirmed  its  relia- 
bility and  superiority  to  the  "  flourometer  "  described  above. 

No  complete  standard  of  fineness  has  yet  been  formulated, 
those  in  use  merely  limiting  the  proportion  of  useless,  coarser 
particles  and  paying  no  attention  to  the  size  of  particle  which 
is  actually  the  most  efficient.  D.  B.  Butler  has  made  experi- 
ments which  appear  to  indicate  that  cement  particles  which 
pass  a  No.  120  sieve,  but  are  retained  on  a  No.  180  sieve,  are 
sufficiently  small,  but  the  demands  of  users  since  those  experi- 
ments were  made  have  resulted  in  the  best  commercial  brands 
of  cements  being  so  fine  that  only  about  5  per  cent,  is  left  on 
a  No.  180  sieve, 


THE  RATE  OF  SETTING  109 

For  the  British  Standard  Specification,  Portland  cement 
shall  be  ground  so  fine  that  the  residue  left  on  a  No.  76  sieve 
must  not  exceed  3  per  cent.,  and  that  on  a  No.  180  sieve  shall 
not  exceed  18  per  cent.  All  the  better  brands  of  Portland 
cement  conform  to  these  limits,  and  a  number  of  them  leave 
only  a  trace  on  the  No.  76  sieve  and  2  per  cent,  or  less  on  the 
No.  180  sieve. 

RATE  OF  SETTING. 

When  Portland  cement  is  mixed  with  water l  a  plastic  paste 
is  formed,  which  soon  loses  its  plasticity,  stiffens  and  "  sets," 
and,  later,  hardens  to  a  stonelike  mass,  which  was  supposed 
by  the  inventor  to  resemble  Portland  stone.  (See  Chapter  IV.) 

Setting  and  hardening  are  two  entirely  different  properties, 
and  seem  to  have  little,  if  any,  connection  with  each  other. 
It  is,  however,  generally  true  that  quick-setting  cements 
harden  more  slowly  than  slow-setting  ones. 

The  speed  at  which  a  cement  sets  when  gauged  with  water 
is  no  criterion  of  its  ultimate  strength,  except  in  so  far  as 
quick-setting  cements  are  very  difficult  to  work  and  so  may 
produce  a  weak  material.  If  the  workman  should  continue 
the  mixing  of  the  cement  and  water  after  the  initial  set  has 
commenced,  the  ultimate  strength  of  the  material  will  be 
seriously  reduced  on  account  of  the  destruction  of  the  crystalline 
network  formed  in  the  first  stage  of  the  setting.  With  modern 
quick-setting  cements,  excessive  gauging  resulting  in  working 
through  the  initial  set  is  responsible  for  much  faulty  work  in 
concrete  construction.  This  is  a  defect  which  is  extremely 
difficult  to  avoid,  and  is  one  of  the  soundest  reasons  for  using 
a  slow-setting  cement  whenever  possible. 

As  previously  mentioned  (p.  81),  two  distinct  stages  are 
recognised  in  the  setting  of  cements :  the  first,  or  initial  set,  is 
when  the  pasty  mass  becomes  just  "  solid,"  and  the  second,  or 
final  set,  when  the  cement  mass  is  sufficiently  hardened  to 
resist  scratching  by  the  thumb-nail  or  by  some  more  accurate 
method  of  applying  a  light  but  definite  pressure,  such  as  a 
Vicat's  needle. 

This  is  terxned  gaugin  g. 


110      TESTING  THE  PROPERTIES  OF  CEMENTS 

The  point  at  which  the  initial  set  occurs  is  often  difficult  to 
recognise  with  quick-setting  cements  unless  some  definite 
method  is  adopted  for  ascertaining  it.  The  one  in  general  use 
consists  in  placing  a  pat  of  the  cement  paste  on  a  glass  plate. 
The  Vicat  needle  is  then  applied  ;  if  it  penetrates  the  pat 
completely  no  setting  has  occurred,  but  if  the  needle  sinks  into 
the  pat,  but  fails  to  penetrate  it,  the  initial  set  has  begun. 

In  stationary  kilns  the  proportion  of  fuel  ash  which  becomes 
mixed  with  the  cement  is  sufficient  to  make  the  latter  slow- 
setting.  Cement  which  has  been  burned  in  rotary  kilns 
contains  much  less  fuel  ash  and  sets  almost  instantaneously, 
unless  a  suitable  amount  of  a  retarding  agent  is  present. 

Cement  manufacturers  supply  quick,  medium  and  slow- 
setting  cements,  and  occasionally  a  lot  may  be  delivered  of 
a  different  rate  of  setting  to  that  to  which  the  user  is 
accustomed. 

Owing  to  the  serious  consequences  which  may  follow  the 
use  of  a  quick-setting  cement  without  its  rate  of  setting  being 
observed,  it  is  important  that  each  bag  of  cement  should  be 
tested  before  use. 

The  three  rates  of  setting  are  defined  in  the  British  Standard 
Specification  as  follows  :— 

Quick. — Initial  setting  time  not  less  than  two  minutes. 
Final  setting  time  not  less  than  ten  minutes,  nor  more 
than  thirty  minutes. 

Medium. — Initial  setting  time  not  less  than  ten  minutes. 
Final  setting  time  not  less  than  half  an  hour,  nor  more 
than  two  hours. 

Slow. — Initial  setting  time  not  less  than  twenty  minutes. 
Final  setting  time  not  less  than  two  hours,  nor  more  than 
seven  hours. 

It  should  be  observed  that  quick-setting  cements  have  a 
lower  tensile  strength  and  a  lower  compressive  strength  than 
those  which  set  more  slowly,  but  this  is  not  invariably  the 
case. 

The  final  set  is  said  to  occur  when  the  Vicat  needle,  having 
been  gently  lowered  on  to  the  pat,  fails  to  make  an  impression 
on  it.  The  needle  should  be  applied  at  sufficiently  frequent 
intervals — usually  every  ten  minutes — to  a  different  part  of 


THE  RATE  OF  SETTING 


111 


Weight 
300  grms. 


the  pat.  Skilled  workers  usually  invert  the  pat  before  testing 
for  the  final  set,  as  the  side  uppermost  when  moulded  is  usually 
covered  with  a  misleading  scum  which  is  much  softer  than  the 
cement. 

The  Vicat  needle  (Figs.  10  and  II)1  ordinarily  used  consists  of 
a  round  steel  bar  which,  with  its  flat  head,  weighs  exactly 
300  grammes.  At  the  lower  end  of  this  rod  a  needle  or  wire 
exactly  1  sq.  mm.  in  cross  section  is  clamped.  The  rod  carries 
an  indicator  which  moves  over  a  graduated 
scale  attached  to  the  frame.  The  cement 
is  held  by  a  split  ring  8  cm.  in  diameter, 
4  cm.  high  (E),  resting  on  a  glass  plate. 

The  cement  confined  in  the  ring  resting 
on  the  plate  is  placed  under  the  rod 
bearing  the  needle,  which  is  then  gently 
brought  into  contact  with  the  surface  of 
the  cement  and  quickly  released  and 
allowed  to  sink  into  the  cement.2  This 
process  is  repeated  until  the  needle,  when 
brought  into  contact  with  the  cement, 
does  not  pierce  it  completely,  and  the 
period  between  the  time  when  the  cement 
is  filled  into  the  mould  and  the  time  at 
which  the  needle  ceases  to  pierce  the 
cement  completely  is  the  initial  setting 
time  above  referred  to. 

Various  auxiliary  devices,  the  object 
of  which  is  to  make  the  Vicat  needle 
automatic  and  self -registering,  have  been 
devised,  but  none  of  them  are  so  satis- 
factory as  the  simpler  form  described.3 

A  test  which  would  be  better  than  the  use  of  a  Vicat  needle 
would  consist  in  measuring  the  pressure  needed  to  drill  a 
standard  distance  into  the  block  of  hardened  cement.  Such 


FIG.  10. — Vicat's 
Needle  (front  view). 


1  The  illustrations  are  of  the  British  Standard  Specification  pattern. 

2  Care  must  be  taken  that  the  needle  C  rests  with  its  full  weight  on  the  pat. 

8  The  Vicat  needle  may,  if  desired,  be  fitted  with  a  mechanical  attachment,  such 
as  a  "  dash-pot."  so  as  to  ensure  the  steady  and  gentle  application  of  the  point  of 
the  needle  to  the  surface  of  the  pat  and  thereby  render  the  test  independent  of  the 
hand  of  the  operator. 


TESTING  THE  PROPERTIES  OF  CEMENTS 


Weight 
^300  grammes 


a  test  would  be  particularly  useful  in  distinguishing  defective 
cements  which  harden  only  on  the  surface  and  leave  a  soft 
interior. 

A  disadvantage  applying  to  all  mechanical  methods  of 
ascertaining  the  time  of  setting  and  hardening  is  the  fact  that 
the  processes  which  occur  in  the  cement  are  chemical,  and  it 
might  be  supposed  that  they  would  be  more  accurately 
measured  by  thermal  than  by  mechanical  methods.  For  this 

reason  it  is  interesting  to  note 
that  a  method  originally  used 
by  Faija,  but  abandoned  by 
him  in  1884,  was  revived  in 
a  modified  form  by  Gary  in 
1906.  H.  Faija  observed  that 
when  the  greased  bulb  of  a 
thermometer  was  placed  in  a 
pat  of  cement  immediately 
after  gauging,  the  tempera- 
ture rose  until  a  maximum 
was  reached,  after  which  it 
slowly  sank  to  the  original 
temperature.  Gary,  however, 
observed  that  at  the  moment 
corresponding  to  the  final  set 
the  temperature  again  rises 
appreciably.1  This  method, 
whilst  apparently  of  great 
promise,  is  affected  by  so 
minor  considerations, 
the  quantity  of  the 


many 
Fio.ll.-VIcaf.  Needle  (side  view).  guch 


material  used  and  the  rate  of  setting,  that  much  further 
investigation  is  necessary  before  it  can  be  brought  into 
general  use.  Moreover,  its  indications  do  not  always  agree 
with  the  generally  accepted  Vicat  needle  test,  the  second  rise  in 
temperature  in  some  cements  occurring  over  an  hour  after  the 
"  final  set  "  shown  by  the  needle.  H.  Faija,  who  investigated 

1  W.  Ostwald,  in  1883,  drew  attention  to  the  rise  in  temperature  5  —  7  days 
after  the  first  set.  W.  and  D.  Asch  consider  that  it  is  due  to  the  separation  and 
hydration  of  calcium  oxide  from  the  alumino-siiicate  molecule. 


THE  RATE  OF  SETTING 


113 


the  thermal  method  very  thoroughly,  found  its  indications 
were  so  irregular  that  he  abandoned  it  in  favour  of  the  needle, 
as  constant  results  can  be  obtained  with  the  latter.  The 
following  table  will  show  the  discrepancies  between  the  two 
methods  : — 


NEEDLE  METHOD. 

THERMAL  METHOD. 

-    Cement. 

Initial 
Set  in 
minutes. 

Final 
Set  in 
minutes. 

Increase 
in  tem- 
perature 
during 

setting. 

Time  to 
reach  first 
maximum 
tempera- 
ture. 

Time  to 
reach 
second 
maximum 
tempera- 
ture. 

A 

1 

5 

39 

5 

110 

B      .         ..'.      . 

8 

17 

33 

12 

224 

C       . 

6 

15 

30 

14 

176 

D      . 

2 

8 

21 

7 

132 

E      . 

8 

23 

25 

11 

114 

F     (overlimed 

and     under- 

burned) 

15 

250 

14 

25 

147 

It  is  very  important,  in  gauging  cements  which  are 
afterwards  to  be  tested,  that  the  water  used  should 
be  free  from  salts,  as  these  would  alter  the  rate  of 
setting. 

The  use  of  well  water  of  exceptionally  low  temperature 
will  cause  test  pieces  to  show  a  low  tensile  and  compressive 
strength,  a  difference  of  only  3  or  4  degrees  below  the 
normal  being  quite  sufficient  for  this  purpose  (p.  131) ;  hence, 
the  necessity  for  testing  the  temperature  of  all  water  used  for 
gauging.  The  mixture  of  cement  and  water,  or  of  cement,  sand 
and  water,  should  be  of  the  proper  consistency.  Unfortunately, 
it  is  very  difficult  to  define  the  limits  of  consistency,  and  the 
amateur  should,  therefore,  make  a  number  of  tests  on  well- 
known  brands  of  cement  and  compare  his  results  with  those 
published  by  the  manufacturer.  In  this  way  he  will  soon 
learn  to  judge  what  is  the  correct  consistency  far  better  than 
by  attempting  to  use  prescribed  limits.  For  the  same  reason, 

c.  i 


114      TESTING  THE  PROPERTIES  OF  CEMENTS 

the  committee  responsible  for  the  British  Standard  Specification 
express  themselves  in  exceedingly  cautious  terms,  merely 
providing  that  "  the  cement  shall  be  mixed  with  such  a 
proportion  of  water  that  the  mixture  shall  be  plastic 


FIG.  12. — Nicol's  Spissograph. 

when   filled   into   the   mould,"    and  adding   the  proviso  that 
"  the    gauging    shall    be    completed    before    signs   of  setting 


occur. 


In  Germany  an  excess  of  water  is  used,  a  syrup  being  first 


THE    SPISSOGRAPH  115 

formed  which  runs  off  the  trowel  in  long  threads.  To  this 
more  cement  is  added  in  small  quantities  with  vigorous 
trowelling  until  the  mixture  slimes  and  ceases  to  adhere  to  the 
mixing  board. 

Where  a  more  accurate  guide  to  the  consistency  of  the  paste 
is  required  than  that  offered  by  noticing  its  behaviour  during 
the  gauging,  the  makers  of  one  form  of  Vicat  needle  (Messrs. 
Adie,  of  London)  provide  a  short  cylinder  1  cm.  in  diameter 
which  replaces  the  ordinary  needle.  The  cement  paste  is 
gauged  until  it  has  reached  what  is  considered  to  be  the 
desired  consistency,  the  time  taken  being  carefully  observed. 
It  is  then  placed  in  the  mould  supplied  with  the  instrument 
and  tested.  The  cylinder  should  sink  to  a  depth  of  6  mm. 
above  the  level  of  the  glass  plate — a  scale  being  provided  on 
the  instrument  to  show  the  depth  it  has  sunk.  If  the  cylinder 
sinks  further  in  the  paste  the  latter  is  too  thin  ;  if  it  does  not 
sink  so  far  the  paste  is  too  thick,  and  fresh  proportions  of  water 
and  cement,  or  cement  and  sand,  must  be  tried  until  the 
correct  consistency  is  obtained. 

An  ingenious  device  for  automatically  recording  the  initial 
and  final  setting  points  is  Nicol's  Spissograph  obtainable  from 
A.  &  J.  Smith,  Maxwell  House,  Aberdeen  (Fig.  12).  This 
consists  of  a  Vicat  needle  suspended  from  a  cord  and  lowered 
on  to  the  cement  at  regular  intervals  by  clockwork.  A 
supplementary  mechanism  ensures  that  the  needle  is  applied 
to  a  different  part  of  the  cement  each  time  it  is  lowered. 
The  depth  to  which  the  needle  sinks  and  the  time  of  each  test 
are  marked  on  a  revolving  chart. 

The  gauging  ought  not  to  occupy  more  than  three  minutes 
with  slow-setting  cements,  or  more  than  one  minute  with 
quick-setting  ones.  It  is  essential  that  the  temperature  of  the 
cement,  water  and  room  in  which  the  gauging  is  performed 
should  be  between  15°  and  16°  C.,  as  variations  in  the 
temperature  have  a  marked  effect  on  the  rate  of  setting. 
Yet  comparatively  little  attention  is  paid  to  this  matter. 
If  the  room  is  too  warm  the  cements  will  be  found  to 
be  quicker  setting  than  they  should  be,  whilst  in  a  cold 
testing  room  a  quick-setting  cement  which  might  cause 
serious  trouble  in  use  may  be  overlooked.  The  following 

I  2 


116      TESTING  THE  PROPERTIES  OF  CEMENTS 


table  by  D.  B.  Butler  shows  the   variations  in   five   typical 
cements  : — 


Temperature  Centigrade. 

Sample 
No. 

38° 

27° 

16°            5° 

38° 

27° 

16° 

5° 

Initial  set  in  minutes. 

Set  hard  in  hours. 

1 

H 

4 

6 

13 

U 

H 

2 

2J 

2 

3 

5 

6 

8 

i 

ii 

If 

2i 

3 

4 

10 

15 

20 

| 

} 

11 

61 

8 

10 

15 

35 

40 

I 

i 

!J 

If 

12 

15 

35 

70 

360 

8f 

6 

7 

22 

The  increased  rate  at  which  cements  set  at  slightly  higher 
temperatures  than  the  normal  makes  it  necessary  in  the 
tropics  to  employ  cements  which  would  here  be  very  slow 
in  setting. 

SOUNDNESS. 

In  order  that  a  cement  may  be  useful  and  satisfactory,  it 
must  not  undergo  any  changes  in  volume  when  in  use  under 
any  probable  conditions  of  exposure.  If  the  cement  shrinks 
unduly  during  setting  it  will  produce  cracks,  whilst  if  it  expands 
after  setting  a  different  kind  of  cracking  is  produced,  which 
is  known  technically  as  "  blowing."  Excessive  contraction  is 
seldom  observed  in  Portland  cements,  the  minute  cracks 
produced  by  the  small  contraction  which  occurs  in  most  cements 
being  almost  entirely  superficial,  and  have  no  appreciable  effect 
on  the  strength  of  the  cement.  Expansion  after  setting 
("  blowing  ")  is,  on  the  contrary,  one  of  the  commonest  defects 
of  badly  made  or  inferior  cements,  and  as  the  results  of  such 
expansion  are  serious,  and  may  even  result  in  the  destruction 
of  a  building  with  loss  of  life,  it  is  of  the  greatest  importance 
to  ascertain  by  appropriate  tests  whether  such  expansion  is 
likely  to  occur.  A  cement  which  cracks  or  twists  after  setting 
is  said  to  be  unsound. 

Unsoundness  is  usually  attributed  to  the  too  tardy  hydration 


THE  SOUNDNESS  OF  CEMENTS  117 

of  some  of  the  constituents  of  a  cement.  It  may,  to  some 
extent,  be  a  result  of  imperfect  mixing  or  gauging  of  the 
cement  with  water,  or  to  the  occurrence  in  the  cement  of 
undesirable  substances.  Thus,  it  is  well  known  that  quick- 
lime has  a  powerfully  expansive  force  when  moistened,  and 
some  other  calcareous  compounds  possess  the  same  property. 
Quite  recently,  however,  Hans  Kuhl  has  found,  experimentally, 
that  normal  Portland  cements  (with  and  without  gypsum), 
dead  burned  lime  and  quick-lime  have  a  smaller  volume  when 
mixed  with  water  than  the  sum  of  the  volumes  of  the  solid 
substance  and  the  water,  the  contraction  being  greater  with 
cements  than  with  quick-lime.  Cements  which  "  blow  " 
badly  after  setting  were  found  by  Kuhl  to  have  a  much  smaller 
contraction  (in  some  cases  they  expanded),  from  which  he 
concludes  that  the  true  cause  of  "  blowing  "  and  expansion 
in  cement  is  to  be  found  in  the  formation  of  crystals  from  a 
supersaturated  solution  and  in  the  pressure  due  to  this 
crystallisation. 

In  former  years  much  of  the  unsoundness  of  cements  was 
due  to  the  use  of  too  much  lime  in  the  raw  materials  ;  this  is 
now  a  far  less  frequent  cause. 

A  cement  in  which  there  is  an  appreciable  proportion  of  free 
lime  or  an  excessive  proportion  of  magnesia  or  of  sulphates  is 
usually  unsound,  and  though  each  of  these  substances  under- 
goes a  different  chemical  reaction  with  water,  the  final  physical 
effect — expansion  and  possible  destruction  of  the  material — 
is  the  same. 

The  action  of  magnesia  in  unsound  cement  is  not  clearly 
understood.  The  strict  limitation  as  to  the  proportion  of 
magnesia  permissible  is  due  to  the  collapse  of  certain  bridges  and 
other  structures,  including  the  Cassel  Town  Hall,  in  which 
magnesian  limestone  was  used  in  the  manufacture  of  the 
cement.  In  these  cases  the  defects  were  attributed  to  the 
magnesia  present,  though  it  is  by  no  means  improbable  that 
this  is  erroneous,  as  excellent  samples  of  cement  have  been 
prepared  from  magnesian  limestone.  It  is,  however,  necessary 
to  burn  cements  containing  magnesia  at  a  higher  temperature 
than  when  no  magnesia  is  present  (see  p.  73). 

Magnesia   requires    a    much    higher   temperature   before   it 


118      TESTING  THE  PROPERTIES  OF  CEMENTS 

combines  with  clay  to  form  a  cement  ;  if  strongly  heated,  yet 
insufficiently  so  to  effect  combination,  it  will  prove  dangerous 
on  account  of  the  great  expansion  of  highly  calcined  magnesia 
in  the  presence  of  water.  Lightly  calcined  magnesia  has  no 
influence  on  cement.  The  increasing  use  of  cements  containing 
a  large  proportion  of  magnesia  indicates  that  the  danger  of 
this  oxide  is  far  less  than  is  commonly  supposed,  if  only  the 
conditions  of  manufacture,  and  particularly  of  burning,  are 
correct.  Improperly  burned  cements — whether  made  of  purely 
calcareous  or  magnesian  limestones — will  be  defective,  the 
latter  being  particularly  so.  Hence  the  limit  of  3  per  cent, 
imposed  in  the  British  specification  is  a  wise  one. 

The  action  of  sulphates  on  cement  is  discussed  later,  in  the 
section  on  concrete,  as  it  is  of  great  importance  in  connection 
with  maritime  work. 

The  presence  of  free  lime  or  of  lime  in  an  unsuitable  state  of 
combination  is  usually  regarded  as  the  chief  and  commonest 
cause  of  unsoundness  in  cements.  Properly  prepared  Portland 
cement  contains  no  free  lime,  but  although  the  original  propor- 
tion of  lime  in  the  cement-mix  may  have  been  correct,  it  is 
not  unusual,  particularly  when  some  of  the  earlier  methods  of 
settling,  drying  and  burning  are  used,  for  the  final  clinker  to 
be  far  from  uniform  in  composition.  With  rotary  kilns  fed 
with  slurry  there  is  less  likelihood  of  the  various  materials 
becoming  unmixed,  but  in  the  intermittent,  stationary  kilns 
fed  with  broken  lumps  of  deposited  material,  the  extent  of  the 
irregularity  is  considerable,  and,  in  some  cases,  is  serious. 
This  irregularity  in  composition  is  partly  due  to  defective 
mixing  appliances  and  partly  to  the  natural  tendency  of 
materials  mixed  in  the  "  wet  process  "  to  "  settle  out  "  in  the 
wash-backs  or  settling  tanks.  No  amount  of  grinding  and 
mixing  of  the  clinker  will  entirely  remove  the  lack  of  homo- 
geneity, for  the  well-mixed  clinker  will  be  composed  partly 
of  true  cement  and  partly  of  calcined  but  uncombined  materials, 
and  can  thus  destroy  the  value  of  the  whole  material.  Under- 
burning  may  also  account  for  the  presence  of  free  lime  in  the 
cement,  and  almost  always  occurs  when  stationary  kilns  are 
used,  owing  to  irregularities  in  the  draught  ;  the  injurious 
action  of  this  is  avoided  by  carefully  sorting  out  the  clinker 


TESTS  FOR  SOUNDNESS  119 

before  sending  it  to  the  mills.  Under-burning  occurs  to  only 
an  insignificant  extent  in  well -managed  rotary  kilns.  Free 
lime  in  Portland  cement  is  seldom  due  to  wrong  proportions 
of  the  raw  ingredients,  special  care  being  taken  to  avoid  this. 
It  may  be  avoided  by  carefully  checking  the  proportions  of 
the  raw  mix  and  by  securing  as  uniform  a  mixing  and  burning 
as  possible,  but  as  there  is  always  a  chance  of  free  lime  being 
present  it  is  advisable  to  test  all  batches  of  cement  as  to  their 
soundness. 

The  general  opinion  that  free  lime  is  the  cause  of  unsoundness 
in  Portland  cements  is  disputed  by  H.  E.  Kiefer,  who  found 
that,  with  sufficiently  fine  grinding,  a  mixture  of  cement  and 
quick-lime  containing  25  per  cent,  of  lime  will  pass  the  ordinary 
tests  for  soundness.  To  avoid  blowing,  all  that  is  necessary 
is  that  the  lime  shall  be  so  finely  ground  that  it  becomes 
hydrated  immediately.  Kiefer's  investigations  seem  to  show 
that  the  phenomenon  of  "  seasoning  "  is  not  so  much  one  of 
hydrating  the  free  lime  as  a  decrepitation  process  in  which 
the  glassy  particles  of  the  cement  are  broken  up.  This  is 
shown  by  the  increased  percentage  of  fine  particles  in  a  cement 
which  has  been  stored  in  vacuo  for  some  weeks.  If  Kiefer's 
views  are  correct  the  widely-held  theory  that  unsoundness  is 
due  to  free  lime  is  not  well  founded. 

The  soundness  of  a  cement  is  difficult  to  ascertain  with 
great  accuracy,  as  the  majority  of  the  Portland  cements  now 
on  the  market  are  of  such  a  character  that  they  all  pass  any 
ordinary  test  for  unsoundness.  A  very  large  number  of 
different  tests  have  been  proposed  as  indicating  the  soundness 
or  otherwise  of  a  cement,  and  some  of  these  are  so  severe  as 
to  make  it  questionable  whether  they  really  measure  the 
soundness  at  all.  In  nearly  all  soundness  tests  the  cement  is 
subjected  to  very  trying  conditions,  and  any  changes  in  it 
(such  as  an  increase  in  volume,  cracking,  etc.)  are  noted. 
A  cement  in  which  no  change  can  be  found  is  regarded  as  sound 
under  the  conditions  of  the  test. 

A  test  which  is  remarkably  accurate,  considering  its  sim- 
plicity, consists  in  gauging  some  of  the  cement  to  be  tested 
with  sufficient  water  to  form  a  slurry  or  cream.  This  is  then 
poured  into  a  test-tube  until  the  latter  is  full.  The  test-tube 


120       TESTING  THE  PROPERTIES  OF  CEMENTS 

with  its  contents  is  hung  in  a  tank  of  cold  water  for  one  or 
more  days.  If  the  cement  expands,  due  to  imperfect  burning 
or  over-liming,  it  will  crack  the  test-tube  ;  if  it  contracts, 
because  the  raw  materials  do  not  contain  sufficient  lime,  the 
cement  will  shrink  and  become  loose  in  the  tube.  Unfortun- 
ately the  test-tube  with  its  contents  cannot  be  immersed  in 
hot  water,  as  the  glass  and  cement  expand  unequally,  and  the 
former  is  cracked  even  with  sound  cements. 

The  contraction  of  cements  after  setting  is  tested  by  means 
of  a  tapered  metal  mould  which  is  filled  with  cement  paste. 
If  the  cement  shrinks  or  contracts  it  will  gradually  become 
loose,  whilst  a  cement  of  constant  volume  will  remain  so  tight 
in  the  mould  as  to  be  difficult  to  remove.  There  is  no  official 
test  for  contraction,  but  the  mould  used  for  determining  the 
rate  of  hardening  is  generally  made  tapered  and  is  commonly 
used  for  ascertaining  whether  a  cement  shrinks. 

Owing  to  the  naturally  slow  hydration  and  other  changes 
which  occur  during  the  hardening  of  cement  it  is  almost 
hopeless  to  expect  that  any  tests  for  soundness  can  be  made 
with  much  rapidity.  Moreover,  it  does  not  necessarily  follow 
that  because  a  cement  can  withstand  the  action  of  boiling 
water  for  several  hours  it  will  therefore  resist  exposure  for 
several  years  ;  in  other  words,  an  accelerated  test  does  not 
necessarily  give  the  same  results  as  actual  use,  during  which 
the  cement  hardens  slowly  in  air.  The  present  tests  must, 
therefore,  be  regarded  as  tentative  in  character.  The  value  of 
such  accelerated  tests  is  disputed  by  many  cement  manufac- 
turers and  chemists,  and  to  such  a  height  did  controversy  rise 
at  one  time  with  regard  to  Le  Chatelier's  test  (p.  125)  that  an 
International  Congress  Committee  was  appointed  to  investigate 
it  thoroughly,  and  found  it  quite  trustworthy  and  capable  of 
detecting  unsound  cements  which  were  passed  as  good  by  a 
number  of  other  tests.  Even  at  the  present  time  the  German 
manufacturers  object  to  this  test,  on  the  ground  that  it  gives 
different  results  with  the  same  cement  when  tested  in  different 
places,  and  that  it  has  failed  to  condemn  some  cements  which 
swell  when  tested  by  the  usual  German  method,  viz.,  keeping 
pats  in  cold  water  for  a  month. 

Although  all  hot- water   tests  do  undoubtedly  reject   some 


HOT  WATER  TESTS  121 

sound  cements,  from  the  user's  point  of  view  there  are  so 
many  firms  manufacturing  cement  which  will  stand  the  action 
of  boiling  water  that  these  naturally  have  the  preference. 
Some  injustice  is  done  to  those  manufacturers  of  sound  cements 
which  will  not  stand  these  tests,  but  under  present  com- 
mercial conditions  this  appears  to  be  unavoidable.  The 
student  should,  however,  always  bear  in  mind  that  a  cement 
is  not  necessarily  unsound  because  it  cannot  stand  the  Le 
Chatelier  test  ;  at  the  same  time,  all  cements  which  do  pass 
this  test  can  be  relied  on  as  being  sound  in  use  so  far  as  the 
existence  in  them  of  expansive  ingredients  is  concerned. 

The  whole  attitude  of  those  responsible  for  the  officially 
recognised  tests  is  somewhat  inconsistent  so  far  as  accelerated 
tests  are  concerned,  and  in  different  countries  widely  differing 
opinions  are  held.  Thus,  in  the  German  Standard  Rules 
recently  issued,  accelerated  tests  are  entirely  ignored,  and  in 
the  American  Standard  Specification  the  following  significant 
paragraph  is  included  :  "In  the  present  state  of  our  knowledge 
it  cannot  be  said  that  cement  should  necessarily  be  condemned 
simply  for  failure  to  pass  the  accelerated  tests  ;  nor  can  a 
cement  be  considered  entirely  satisfactory  simply  because  it 
has  passed  these  tests." 

Whatever  excellent  reasons  there  may  be  for  or  against 
accelerated  tests,  it  is  clearly  unwise  for  engineers  and  others 
engaged  in  the  use  of  cement  to  be  too  dogmatic  on  the  subject. 
It  is  a  well-known  fact  that  fifteen  years  ago  not  10  per  cent,  of 
Portland  cement  manufactured  in  England  or  elsewhere  would 
withstand  boiling- water  tests,  and  the  obvious  inference  is 
that  if  these  are  a  true  test  for  soundness,  90  per  cent,  of  the 
cement  used  fifteen  years  ago  was  unsound.  This,  in  view  of 
the  hundreds  of  thousands  of  tons  then  used  for  important 
engineering  work  throughout  the  world,  is  rather  startling  and, 
having  regard  to  the  excellent  condition  of  such  work  at  the 
present  time,  is  a  view  that  cannot  be  seriously  maintained. 
The  risk  of  using  unsound  cement  is  so  great,  however,  that 
it  is  questionable  whether  any  test  can  be  really  too  severe, 
providing  that  it  is  a  true  test  for  soundness  and  not  for  some 
unimportant  property. 

The  earliest  method  of  testing  the  soundness  of  a  cement 


122       TESTING  THE  PROPERTIES  OF  CEMENTS 

consisted  in  making  the  cement  paste  into  a  thin  pat  with 
tapering  edges  and  in  placing  it  in  water  as  soon  as  it  is  set. 
If  at  the  end  of  a  week  it  developed  no  cracks  or  twists  it  was 
considered  to  be  sound.  This  is  known  as  the  "  plunge  test," 
but  is  now  seldom  used  ;  it  has  been  found  in  practice  to  be 
too  lenient,  as  exposure  for  a  longer  period  frequently  developed 
cracks,  and  it  is  unfair  to  cements  which  set  very  slowly.  It 
has,  therefore,  been  modified  to  avoid  this  objection.  Thus, 
in  what  is  known  as  the  "  cold  water  test,"  the  cement  pat 
is  kept  for  twenty-four  hours  in  moist  air  and  is  then  placed 
in  water  for  twenty-eight  days.  This  modification  requires 
an  inconveniently  long  time,  and  is  considered  to  be  too  lenient  ; 
its  severity  has  therefore  been  increased  in  various  ways.  At 
the  other  extreme  is  Erdmenger's  test,  in  which  the  test 
pieces  are  heated  in  an  autoclave  under  a  pressure  of  560  Ibs. 
per  square  inch  ;  this  test  has  been  regarded  as  unnecessarily 
severe. 

Of  the  accelerated  tests,  that  devised  by  H.  Faija  was 
exceedingly  popular  for  some  years.  It  consists  in  main- 
taining a  freshly  gauged  pat  in  water  vapour  at  a  temperature 
of  38°  to  40°  C.  for  about  seven  hours,  or  until  thoroughly 
set,  and  then  immersing  it  in  water  of  a  temperature  of  46°  to 
49°  C.  for  the  remainder  of  the  twenty-four  hours.  A  sketch 
of  the  apparatus  used  is  shown  in  Fig.  13.  It  consists  of  a 
double-walled  vessel  in  which  the  space  between  the  walls  is 
filled  with  water  to  act  as  a  temperature  equaliser.  The 
inner  vessel  is  only  partially  filled  with  water  so  that  pats  of 
cement  placed  on  the  shelf  shown  are  immersed  in  vapour. 
According  to  D.  B.  Butler,  who  has  an  exceptionally  thorough 
acquaintance  with  this  test,  a  cement  can  be  relied  upon  with 
perfect  confidence  if,  after  being  treated  in  the  manner 
described,  it  shows  no  signs  of  cracking  or  blowing  at  the  end 
of  twenty-four  hours  and  adheres  firmly  to  the  glass  plate  on 
which  it  was  made.  Butler  insists  that  the  narrow  limits  of 
temperature  prescribed  are  essential  ;  if  a  lower  temperature 
than  46°  C.  is  permitted  a  faulty  cement  will  go  undetected, 
whilst  if  exposed  to  too  high  a  temperature  some  cements  will 
be  condemned  which  would  prove  satisfactory  in  use.  The 
great  advantage  of  the  Faija  test  is  that  it  requires  only 


HOT  WATER  TESTS 


123 


twenty-four  hours  to  sort  out  almost  all  the  defective  cements 
submitted  to  it,  and  where  testing  machines  and  plant  are  not 
available  it  is  generally  quite  satisfactory.  During  the  past 
few  years,  however,  the  demand  for  more  severe  tests,  in  which 
the  cement  is  exposed  to  the  action  of  boiling  instead  of  merely 
warm  water,  has  resulted  in  the  Faija  test  falling  into  disuse, 
the  Le  Chatelier  test  being  included  in  the  British  Standard 
Specification. 

The  most  characteristic  feature  of  Faija's  test,  viz.,  exposure 
of  the  freshly-made  pat  to  warm  moist  air  in  order  to  accelerate 
the  setting  and  hardening,  has 
been  abandoned  in  more  recent 
tests  in  favour  of  a  longer 
exposure  to  cold  moist  air. 
This  is  regarded  by  many  of 
those  interested  in  cement  as 
unfortunate,  for  extensive  ex- 
periments have  shown  that 
the  higher  temperature  has  a 
very  important  effect  on  the 
cement. 

DevaVs  Hot  Water  Test  has 
also  been  the  subject  of 
much  controversy.  It  con- 
sisted originally  in  allowing 
a  pat  of  cement  to  remain 
in  moist  air  for  twenty-four 
hours  and  then  immersing  it 
in  hot  water  at  a  temperature  of  80°  C.  for  six  days 
or  more,  after  which  treatment  sound  cements  are  to 
show  no  signs  of  twisting,  cracking  or  blowing.  The  tensile 
strength  of  test  pieces  subject  to  Deval's  test  is  equal  to  that 
gained  after  twenty-seven  days'  immersion  in  cold  water. 
More  recently,  the  temperature  of  the  water  used  in  the  test 
has  been  raised  to  boiling  point,  and  the  time  has  been  shortened 
to  three  hours,  both  of  which  modifications  will  greatly  simplify 
the  test.  In  this  modified  form  the  test  is  much  used  in 
Germany  under  the  somewhat  inappropriate  term,  "  Darr- 
probe,"  which  really  signifies  a  drying  test.  Twelve  or  fourteen 


jfe 


FIG.  13. — H.  Faija's  Test  for 
Soundness. 


124      TESTING  THE  PROPERTIES  OF  CEMENTS 

years  ago  Deval's  test  was  considered  to  be  unduly  severe,  but 
so  greatly  has  the  manufacture  of  cement  improved  within  the 
last  decade  that  in  1909  the  International  Association  for 
Testing  Materials  found  that  all  good  cements  stood  it  easily, 
but  that  it  was  uncertain  in  indicating  some  doubtful  ones. 
On  the  recommendation  of  this  association  it  was  therefore 
abandoned  in  Great  Britain  in  favour  of  the  Le  Chatelier  test. 

Bauschinger's  Method. — One  of  the  most  accurate  methods 
of  measuring  the  expansion  or  contraction  of  cement  is  that 
devised  by  Bauschinger,  who  uses  a  special  micrometer  calliper, 
in  which  the  test  pieces  consist  of  prisms  or  square  bars  100  mm. 
long  and  22  mm.  by  22  mm.  cross  section  ;  the  delicacy  of  the 
instrument  is  such  that  variations  in  the  length  of  the  bar  to 
within  2Uo  mm.,  or  0-0005  per  cent.,  can  be  determined  with 
certainty.  It  requires,  however,  such  very  careful  expert 
handling,  and  is  somewhat  expensive  (the  equipment,  including 
moulds,  costing  about  £12),  that  it  is  only  used  for  research  pur- 
poses when  an  unusually  high  degree  of  exactitude  is  required. 

Le  Chatelier' s  Test  is  based  on  a  somewhat  different  principle 
to  those  previously  mentioned.  Instead  of  the  treated  samples 
being  examined  for  cracks  they  are  measured  before  and  after 
heating  in  water  and  the  amount  of  expansion  is  noted.  Le 
Chatelier's  test  is  therefore  much  more  sensitive  than  the 
earlier  ones,  and  is  correspondingly  more  severe.  The  total 
increase  in  volume  is  exceedingly  small,  and  is  almost  impossible 
of  direct  measurement  except  with  the  use  of  exceedingly 
delicate  appliances,  which  are  more  suited  to  the  purposes  of 
scientific  research  than  to  the  needs  of  the  cement  manufacturer 
and  user.  This  difficulty  has,  however,  been  overcome  in  a 
very  ingenious  manner  by  the  invention  by  Le  Chatelier  of  a 
special  calliper  (Fig.  14)  which  greatly  magnifies  the  expan- 
sion. Le  Chatelier's  calliper  x  consists  of  a  brass  cylinder  0-5 
mm.  (-02  inch)  in  thickness  forming  a  mould  30  mm.  (1T3^  inch) 
internal  diameter,  and  30  mm.  (1T36  inch)  high.  This  ring  is 
split,  and  on  each  side  of  the  split  an  indicator  165  mm. 
(6J  inches)  long  is  attached.  The  free  ends  of  these  indicators 
are  pointed  so  as  to  facilitate  accurate  and  rapid  reading. 

1    This    description    of    the   instrument    and   test    is    taken   from   the   British 
Standard  Specification. 


LE  CHATELIER'S  EXPANSION  TEST 


125 


In  carrying  out  the  test  the  calliper  is  placed  on  a  small 
sheet  of  glass  and  is  filled  with  freshly-gauged  cement,  the 
edges  of  the  cylinder  being  kept  together  during  this  operation 
by  means  of  a  clip  or  piece  of  fine  string.  The  cylinder  is  then 
covered  with  another  piece  of  glass  on  which  a  small  weight  is 
placed,  and  the  whole  appliance  is  then  placed  in  cold  water 
(15°  C.)for  twenty-four  hours.  The  calliper  is  then  taken  out 
of  the  water,  the  clip  or  string  fastening  removed,  and  the 
distance  apart  of  the  points  of  the  indicators  is  accurately 
measured,  and  the  calliper  with  its  contents  is  then  placed  in 
cold  water  which  is  heated  at  such  a  rate  that  it  boils  in  about 
half  an  hour  and  is  kept  boiling  for  six  hours.  The  calliper  is 


^A 


Split  cylinder  of  spring  brass  or  other 
suitable  metal  about  Vzm/m  in  thickness 


Glass 


i  Glass 


.    FIG.  14.— Le  Chatelier's  Test. 

again  removed  from  the  water,  allowed  to  cool,  and  the  distance 
apart  of  the  pointed  ends  of  the  indicators  is  again  measured. 
The  increase  in  their  distance  is  proportionate  to  the  expansion 
of  the  cement.  The  British  Standard  Specification  imposes 
the  following  limits  of  expansion  under  the  foregoing 
conditions  :  The  difference  between  the  two  measurements 
must  not  exceed  the  following  limits,  namely,  10  mm.  when 
the  cement  has  been  spread  in  a  layer  three  inches  thick  and 
exposed  to  the  air  for  twenty-four  hours,  or,  if  this  fails, 
5  mm.  after  the  cement  has  been  exposed  to  the  air  for  seven 
days  in  the  same  manner. 

In  cold  water,  iron  cements  expand  rather  more  than  Portland 


126       TESTING  THE  PROPERTIES  OF  CEMENTS 

cements,  a  mixture  of  hydraulic  lime  and  sand  (1:3)  shows  the 
same  expansion  as  Portland  cement,  whilst  a  mixture  composed 
of  four  parts  of  trass,  three  of  lime  and  two  of  sand  only  expands 
half  as  much  as  Portland  cement.  Roman  cements  usually 
show  an  expansion  of  20  mm.,  and  hydraulic  limes  an  expansion 
of  4  mm.  in  the  Le  Chatelier  test.  None  of  these  substitutes 
for  Portland  cement  can  withstand  long  exposure  to  boiling 
water  ;  they  crack  and  disintegrate. 

As  previously  mentioned  (p.  120),  Le  Chatelier's  test  is  by 
no  means  generally  accepted,  notwithstanding  its  inclusion  in 
several  standard  specifications.  It  yields  some  curiously 
anomalous  results  at  times,  such  as  greater  expansion  in  cold 
water  than  after  boiling  for  six  hours.  Some  of  the  difficulties 
experienced  in  its  use  are  undoubtedly  due  to  the  delicacy  of 
the  measurements  to  be  made  ;  others  are  due  to  an  insufficient 
allowance  for  certain  characteristics  in  the  cements  themselves. 
Thus,  cements  which  set  very  slowly  will  yield  bad  results  if 
tested  in  the  ordinary  manner  by  Le  Chatelier's  method,  but 
if  the  same  cements  are  allowed  to  harden  properly  before 
being  tested  the  results  will  be  normal,  i.e.,  there  will  be  only 
a  trifling  increase  in  volume.  This  test  is,  therefore,  unsuitable 
for  cements  which  do  not  set  hard  in  twenty-four  hours.  The 
results  of  the  Le  Chatelier  test  should  not,  for  these  reasons, 
be  interpreted  too  rigidly. 

The  soundness  of  cement  is  increased  by  fine  grinding,  but 
in  many  cases  users  do  not  avail  themselves  of  the  extra 
fineness  (which  should  enable  them  to  use  less  cement),  but 
continue  to  use  the  same  proportion  as  in  years  gone  by  when 
only  coarse  cements  were  available. 

COMPRESSIVE  STRENGTH. 

The  value  of  a  cement  depends  chiefly  on  its  power  to  bind 
particles  of  inert  material  together  so  as  to  form  a  compact 
mass  of  great  strength.  The  chief  mechanical  resistance 
required  by  structural  materials  in  which  cement  is  used  is 
that  to  compression,  and,  accordingly,  the  compressive  strength 
of  cement,  and  more  particularly  of  concrete  or  cement-sand 
mortar,  is  of  the  greatest  importance. 


COMPRESSIVE  STRENGTH  OF  CEMENTS        127 

The  compressive  strength  of  a  cement,  concrete  or  mortar  is 
ascertained  by  crushing  cubes  of  the  material  in  a  hydraulic 
press  (Fig.  93) ,  the  pressure  applied  being  measured  by  means 
of  a  sensitive  gauge.  The  cubes  usually  measure  three  inches,  or 
70-7  mm.,  each  side  for  cement,  and  6-inch  cubes  for  concrete. 

The  Tests  Committee  of  the  Concrete  Institute  specify  a 
crushing  test  in  addition  to  the  tests  in  the  British  Standard 
Specification.  This  test  is  carried  out  as  follows  :  3-inch 
cubes  consisting  of  three  parts  by  weight  of  standard  sand 
to  one  part  of  cement  shall  be  made  in  the  manner  described 
for  cement-sand  mixtures  used  in  the  tensile  test.  The  crushing 
strength  shall  not  be  less  than  ten  times  the  tensile  strength 
after  twenty-eight  days  required  by  the  British  Standard 
Specification. 

It  is  difficult  to  obtain  uniform  results  in  tests  of  the  com- 
pressive strength  of  cement,  as  apparently  trifling  differences 
in  the  arrangement  of  the  cubes  or  in  the  distribution  of  the 
pressure  produce  marked  variations  in  the  result.  To  equalise 
the  pressure  as  far  as  possible,  thin  boards  of  soft  wood  are 
placed  above  and  below  the  cube  to  be  tested.  If  the  crushing 
strain  has  been  properly  distributed  the  cube  will,  when  crushed, 
leave  two  fairly  perfect  pyramids. 

Tests  of  compressive  strength  form  part  of  the  official  tests 
on  the  Continent,  but  in  Great  Britain  they  are  seldom  made, 
the  difficulties  in  the  way  of  obtaining  concordant  results  being 
considered  to  be  too  great  for  the  test  to  be  brought  into 
general  use.  The  unsatisfactoriness  of  the  results  of  tensile 
tests  and  the  obvious  advantages  of  compressive  tests  are, 
however,  becoming  more  generally  recognised,  and  it  is  probable 
that  before  many  years  a  minimum  compressive  strength 
(probably  250  Ibs.  per  square  inch)  will  be  included  in  the 
British  Standard  Specification.  However,  it  has  been  found 
that  the  tensile  strength  is  so  closely  proportionate  to  the  com- 
pressive strength  in  well-made  cements,  being  usually  one- 
tenth1  of  the  latter,  and  tests  of  tensile  strength  are  so  much 
easier  to  make,  that  they  are  generally  substituted  for  com- 
pressive tests. 

1  The  ratio  of  tensile  to  compressive  strength  increases  with  time,  so  that  in  very 
old  mortars  it  may  be  as  high  as  1-18. 


128      TESTING  THE  PROPERTIES  OF  CEMENTS 


TENSILE  STRENGTH. 

At  the  present  time,  the  test  which  is  regarded  as  of  the 
chief  importance  is  that  of  tensile  strength.  Although  it  is 
very  convenient,  the  test  itself  is  so  illogical  (cement  structures 
seldom,  if  ever,  being  subjected  to  tensile  stresses)  and  the 
results  are  so  erratic  that  it  cannot  be  regarded  as  really 
satisfactory.  It  is  commonly  supposed  that  the  tensile 
strength  is  directly  proportionate  to  the  compressive  strength 
of  the  material,  but  this  ratio  is  only  approximate  and  varies 
considerably  with  different  cements.  This  has  been  fully 
established  by  results  obtained  by  Tetmajor.  Hence,  the  test 

of  tensile  strength  is 
only  used  because  of 
its  general  conveni- 
ence and  should  be 
replaced  by  com- 
pressive tests  when- 
ever possible. 

As  will  be  under- 
s tood  from  the 
statements  made  in 
respect  of  setting 
and  hardening  in 
140  weeks,  a  previous  chapter, 
cements  gradually 


Ibs/sq.fn 
600 


4-00 


200 


20         4.0        60         80         100       120 

FIG.  15. — Increase  of  Tensile  Strength  with 
Time  (Unwin). 


increase  in  strength 
as  they  increase  in  hardness.  The  increase  is  very  rapid  at  first, 
but  becomes  increasingly  slow  with  the  age  of  the  gauged 
material,  and  though  it  does  not  reach  a  complete  maximum, 
even  after  several  years,  the  strength  reached  after  two  years 
is  so  near  to  the  final  maximum  that  it  may  be  regarded  as 
identical  for  most  purposes.  The  chemical  and  physical  changes 
which  occur  during  hardening  are  extremely  complex,  and  it  is 
therefore  necessary  to  determine  the  strength  of  a  cement  on  at 
least  two  different  dates,  which  should  be  as  widely  separated 
as  possible.  From  a  number  of  such  tests  a  graph  may  then 
be  drawn  and  the  probable  maximum  strength  of  the  cement 
ascertained  by  extrapolation.  Two  such  graphs  obtained  by 


TESTING  TENSILE  STRENGTH  129 

W.  C.  Unwin,  in  1886,  are  shown  in  Fig.  15.  The  increase  in 
strength  is  so  rapid  during  the  first  week  that  it  cannot  be  used 
for  estimating  the  maximum  strength  likely  to  be  developed  ; 
after  the  expiration  of  a  year  the  rate  of  increase  is  very  slow, 
and  the  total  strength  then  approaches  the  maximum. 

It  will  be  thus  understood  that  the  total  strength  is  likely 
to  be  greatest  when  the  rate  of  increase  does  not  diminish 
very  rapidly  during  the  weeks  following  the  first.  Hence,  the 
custom  has  arisen  of  ascertaining  the  tensile  strength  at  the  end 
of  three,  seven  and  twenty-eight  days,  respectively,  from  the 
time  of  gauging.  Much  longer  intervals  are  desirable,  but  are 
impracticable  under  most  circumstances,  and,  so  far,  the  three 
periods  just  mentioned  appear  to  give  a  sufficient  clue  as  to 
the  probable  maximum  strength.  It  is  not  desirable  to 
calculate  the  maximum  strength  likely  to  be  developed,  but 
to  see  that  the  strength  at  certain  periods  after  gauging  is 
above  certain  minimum  limits.  The  most  suitable  limits 
recognised  at  the  present  time  are  prescribed  in  the  standard 
specifications  of  the  chief  civilised  countries  where  Portland 
cement  is  used.  Thus,  the  British  Standard  Specification 
demands  that  test  pieces  (erroneously  termed  "  briquettes  "), 
made  as  directed  (see  p.  132)  and  tested  in  a  suitable  machine, 
in  which  the  load  is  applied  steadily  at  the  rate  of  100  Ibs.  in 
twelve  seconds,  shall  give  the  following  results  :— 

FOR  NEAT  l  CEMENT  : 

25  per  cent,  when  the  seven-day  test  is  above  400  Ibs.  and  not  above  450  Ibs. 
20        „  „  „  „         450    „  „  500    „ 

15        „  „  „  „         500    „  „  550    „ 

10        „  „         550    „  „  600    „ 

5        „  „  „  „         600    „ 

FOR  1  :  3  CEMENT-SAND  MORTAR  : 

25  per  cent,  when  the  seven-day  test  is  above  200  Ibs.  and  not  above  250  Ibs. 
15        „  „  „  „         250    „  „  300    „ 

10        „  „  „  „         300    „  „  350    „ 

5        „  „  „  „         350    „ 

Measurements  of  the  tensile  strength  of  test  pieces  which 
have  been  "  aged  "  artificially  by  immersion  in  hot  water  1^-ve 

1  By  neat  cement  is  meant  a  mixture  of  cement  and  water  alone,  no  sand  or  other 
aggregate  being  added.  The  objections  to  tests  of  the  tensile  strength  of  neat 
cements  are  stated  later  under  sand-cement  tests. 

C.  K 


130      TESTING  THE  PROPERTIES  OF  CEMENTS 

not  given  satisfactory  results,  the  strength  of  the  "  accelerated  " 
test  pieces  never  showing  any  definite  relation  to  those  of  the 
same  cements  kept  under  normal  conditions  for  more  lengthy 
periods.  Attempts  to  measure  the  strength  of  "  accelerated  " 
test  pieces  have  now  been  abandoned.  Accelerated  tests, 
omitting  the  determination  of  the  tensile  strength,  are,  however, 
valuable  for  indicating  the  presence  of  expanding  constituents 
(p.  120). 

The  complexity  of  the  reaction  which  occurs  during  the 
hardening  of  cement  is  so  great  that  the  utmost  care  must  be 
taken  in  gauging  the  cement  (or  cement-sand  mixture)  with 
water  if  uniform  results  are  to  be  obtained.  For  the  same 
reason  at  least  six  test  pieces  must  be  used  for  each  test, 
as  single  tests  are  often  far  from  correct.  The  following 
precautions  are  essential  :— 

(a)  The  proportion  of  water  used  must  be  correct  within  somewhat 
narrow  limits.  This  has  already  been  mentioned  with  regard 
to  the  setting  and  hardening  (soundness)  of  cements  (p.  113), 
but  it  is  particularly  important  in  connection  with  the  tensile 
tests.  No  definite  proportions  of  water  can  be  specified,  as 
cements  differ  greatly  in  the  amounts  they  require.  It  may, 
however,  be  taken  that  the  best-known  brands  of  Portland 
cement  require  about  18  to  25  per  cent,  of  water  for  neat 
cement,  and  about  12  per  cent,  for  the  usual  3  :  1  mixture  of 
sand  and  cement.  The  smaller  the  proportion  of  water  the 
better  will  be  the  results.  Some  amount  of  experience  and 
skill  is  needed  to  know  whether  sufficient  or  too  much  water 
has  been  added.  .  If  the  former,  the  cement  will  crumble 
under  the  trowel  used  for  mixing  it  and  cannot  be  made  to 
take  a  smooth  surface  ;  with  too  much  water,  on  the  contrary, 
the  paste  will  be  so  fluid  that  it  can  be  poured  from  one  vessel 
to  another.  It  will  usually  be  found  that  if  the  water  just 
rises  to  the  surface  of  the  cement  when  the  mixture  has  been 
well  smoothed  with  a  trowel,  the  mixture  has  the  right  con- 
sistencv  'It  will  then  be  plastic  without  being  unduly  dry 
^2  ifuid.  This  plastic  mass  can  be  mixed  and  moulded  to  the 
greatest  advantage  and  is  therefore  preferred  to  a  drier  mixture 
possibly  containing  only  10  per  cent,  of  water,  though  the 
latter  may  give  a  stronger  product.  Such  dry  mixtures  give 


TESTING  TENSILE  STRENGTH  131 

such  irregular  results,  however,  that  they  cannot  be  relied 
upon. 

The  British  Standard  Specification  gives  no  precise  directions 
as  to  the  proportion  of  water,  but  merely  states  that  it  must 
be  such  that  "  after  filling  into  the  mould  the  mixture  shall  be 
plastic." 

(b)  The  temperature  of  both  water  and  cement  must  be  normal, 
i.e.,    between  14J0    and    18°  C.  (58°  to  64°  F.).     If   the   tem- 
perature of  the  water  is  only  10°  C.  above  normal  the  strength 
of  the  cement  may  be  reduced  20  per  cent. 

Lack  of  increase  in  strength  on  storage  is  also  frequently 
traceable  to  the  cement  being  tested  in  too  hot  a  room,  wherein 
the  test  pieces  harden  so  rapidly  that  they  really  form 
accelerated  tests.  This  trouble 
is  particularly  noticeable  in 
summer  in  buildings  with  iron 
or  glass  roofs. 

(c)  The  duration  of  the  gauging 
must    be   controlled.      Steinbriick 
has  found  that  the  strength   of 
cement      increases       with       the 
thoroughness    and    duration    of 
the    gauging    so     long     as     the 

commencement  of  the  initial  set     FlG"  16—Fai^Mechanical 
is    not    reached.      This    merely 

means  that  the  better  the  mixing  the  stronger  will  be  the 
cement,  and  for  this  reason,  and  because  the  gauging  of  many 
cements  becomes  extremely  fatiguing,  the  author  has  made 
extensive  use  of  a  simple  mechanical  gauger  devised  by 
H.  Faija  (Fig.  16).  This  consists  of  a  circular  pan  about  one 
foot  in  diameter,  within  which  revolve  the  arms  of  a  stirrer. 
These  arms  revolve  round  their  own  axis  in  one  direction  and 
round  the  pan  in  the  reverse,  this  motion  being  given  them  by 
an  internally  toothed  wheel,  which  actuates  the  pinion  of  the 
stirring  spindle.  The  modus  operandi  is  as  follows  :  After 
having  ascertained,  by  means  of  a  preliminary  hand-gauged 
pat,  how  much  water  the  cement  under  treatment  requires, 
sufficient  cement  to  fill  a  nest  of  moulds  is  put  into  the  gauger, 
and  the  correct  amount  of  water  added  all  at  once.  The 

K  2 


132       TESTING  THE  PROPERTIES  OF  CEMENTS 


handle  of  the  machine  is  then  turned  fairly  quickly  for  a  half 
or  three-quarters  of  a  minute,  by  which  time  it  will  be  found 
that  the  cement  is  thoroughly  incorporated  with  the  water, 
and  the  mass  is  in  a  proper  condition  to  be  turned  out  on  the 
gauging  plate  or  bench  and  filled  into  the  moulds.  In  gauging 
cement  and  sand  in  this  machine,  for  making  sand  briquettes,  it 
is  necessary,  of  course,  first  thoroughly  to  mix  the  sand  and 


approximately 
0-10  approximately 


FIG.  17. — Dimensions  of  British  Standard  Test  Piece. 

cement  in  the  dry  state,  after  which  the  same  routine  may  be 
followed.  By  the  use  of  this  machine  two  or  three  pounds  of 
cement  at  a  time  can  be  efficiently  gauged  in  a  few  minutes, 
and  personal  experience  has  proved  it  to  be  of  the  greatest 
value  in  avoiding  the  labour  and  wrist  work  necessary  to  bring 
the  cement  to  a  proper  consistency  with  a  trowel. 

The  whole  operation  of  filling  a  nest  of  moulds,  from  the 
time  of  adding  the  water,  should  not  exceed  five  or  six  minutes, 
and  after  being  smoothed  off  with  a  trowel,  the  moulds  should 
be  placed  on  one  side  until  the  briquettes  are  sufficiently  set 
to  be  removed. 


TESTING  TENSILE  STRENGTH  133 

During  recent  years  the  Steinbriick-Schmelzer  machine  has 
been  brought  into  extensive  use  for  gauging  cement  for  testing 
purposes.  This  machine  consists  chiefly  of  a  single  edge- 
runner  moving  in  a  rotating  annular  trough.  Both  roller  and 
trough  move  in  the  same  direction  but  at  different  speeds,  the 
paste  being  turned  over  by  two  curved  scrapers.  The  action 
of  this  machine  is  very  satisfactory,  about  twenty  rotations  of 
the  pan  being  ample  to  mix  the  cement.  The  roller  and 
scrapers  are  then  moved  out  of  the  way,  giving  ready  access 
to  the  paste,  and  permitting  of  a  rapid  cleaning  of  the 
machine. 

If  a  cement  is  very  quick  setting  it  may  be  necessary  to 
gauge  it  by  hand  in  very  small  quantities  at  a  time,  or  it  may 
be  spread  out  in  a  thin  layer  and  exposure  to  the  air  to  aerate 
for  two  or  three  days  ;    this 
exposure  will  make  it  slower 
setting.      The    addition    of 
any   retarding    agent   to    a 
cement  for  the  purpose  of 
facilitating    the    testing    is 
expressly  forbidden  in  the 
British  Standard  Specifica- 
tion.    On  no  account  must    FIG.  18. — Mould  for  Testing  Cement, 
the   gauging   be   continued 

after  the  initial  set  has  commenced,  or  the  strength  of  the 
material  will  be  irretrievably  reduced.  In  gauging  cements 
of  which  the  rate  of  setting  is  unknown  it  is,  therefore,  desirable 
to  test  the  rate  of  setting. as  described  on  p.  109. 

(d)  The  mould  must  be  of  standard  size  and  shape.     That  now 
universally  adopted  was  devised  by  Grant,  but  the  shape  of  the 
ends  has  been  modified.    Fig.  17  shows  the  standard  dimensions, 
and  Fig.   18  the  mould.     The  mould  and  plate  on  which  it 
stands  should  be  slightly  oiled  before  use. 

(e)  The  moulds  must  all  be  filled  in  a  uniform  manner.     The 
usual  method  consists  in  placing  the  mould  on  a  glass  or  smooth 
metal  plate,  taking  up  on  a  trowel  more  paste  than  will  fill 
the  mould,  throwing  the  paste  into  the  latter,  and  then  tamping 
it  with  the  trowel  until  the  mould  is  filled  evenly,  and  the 
water  rises  to  the  surface,  giving  it  a  shiny  appearance.     The 


134      TESTING  THE  PROPERTIES  OF  CEMENTS 

superfluous  paste  is  then  cut  off  by  drawing  the  edge  of  the 
trowel  across  the  top  of  the  mould. 

(/)  The  test  piece  must  be  stored  under  standard  conditions. 
It  is  important  that  the  moulds  and  their  contents  should  be 

PLAN 


i-oo 


ELEVATION 


L_.0-7f..JL i  20 -*.-  075 


FIG.  19. — Jaws  of  Tensile  Machine. 

placed  on  a  bench  which  is  free  from  vibration,  or  the  strength 
of  the  test  pieces  will  be  impaired.  If  kept  on  the  same  bench 
or  table  as  that  used  for  filling  other  moulds,  low  results  will 
be  obtained,  and  too  much  care  cannot  be  taken  in  this  respect, 
particularly  when  tests  are  made  of  mixtures  of  cement  and 


TESTING  TENSILE  STRENGTH 


135 


sand.  It  should  also  be  noted  that  all  test  pieces  used  for 
strength  tests  should  be  kept  immersed  in  water  until  ready  for 
testing.  This  keeps  them  at  a  uniform  temperature  and  also 
makes  the  test  rather  more  severe  than  when  the  test  pieces 
are  kept  in  air. 


Fi<?.  20. — Tensile  Test  Machine.     (Adite.) 

The  British  Standard  Specification  demands  that  when  the 
cement  has  set  sufficiently  to  enable  the  test  piece  to  be 
removed  from  the  mould  without  injury,  it  is  so  removed  and 
kept  in  a  damp  atmosphere  for  the  remainder  of  the  twenty- 
four  hours  after  gauging.  It  is  then  to  be  placed  in  clean  fresh 
water  and  allowed  to  remain  there  until  required  for  breaking. 


136      TESTING  THE  PROPERTIES  OF  CEMENTS 

The  water  used  for  this  purpose  is  to  be  changed  every  seven 
days,  and  is  to  be  maintained  throughout  at  a  temperature  of 
14J0  to  18°  C.  (58°  to  64°  F.). 

The  test  pieces  are  to  be  tested  for  tensile  strength  at  seven 
and  twenty-eight  days  after  gauging,  six  pieces  being  used 
for  each  period. 

(g)  The  jaws  of  the  testing  machine  must  be  of  standard  size 
and  shape.  Those  most  generally  used  are  the  ones  recognised 
by  the  British  Standard  Specification,  and  shown  in  Fig.  19. 
The  test  pieces  are  greased  slightly  where  they  come  in  contact 
with  the  jaws. 


I 

FIG.  21.—  Tensile  Test  Machine.     (8 alter  <fr  Co.) 

The  testing  machine  may  be  of  any  convenient  pattern  ; 
one  of  the  most  compact  is  the  compound  lever  originally 
devised  by  Adie  (Fig.  20),  but  several  other  patterns  are  in 
use.  So  long  as  the  load  is  applied  "  steadily  and  uniformly, 
starting  from  zero  and  increasing  at  the  rate  of  100  Ibs.  in 
twelve  seconds  "  the  particular  type  of  machine  is  unim- 
portant. 

Some  means  of  arresting  the  indicator  or  of  enabling  the 
machine  to  work  automatically  is  desirable.  In  the  machine 
shown  in  Fig.  21,  the  weight  corresponding  to  the  breaking 
strain  is  applied  in  the  form  of  shot,  which  runs  from  a  reservoir 


TESTING  TENSILE  STRENGTH  137 

into  a  vessel  attached  to  the  beam  of  the  machine.  When 
the  test  piece  breaks,  this  vessel  falls,  strikes  a  lever  below  it 
and  instantly  cuts  off  the  supply  of  shot.  The  shot  in  the 
receiving  vessel  is  then  weighed  and  multiplied  by  the  necessary 
factor  to  express  the  breaking  strain  in  pounds  per  square  inch. 
In  some  testing  machines,  water  is  used  instead  of  shot,  and 
in  others  a  weight  (Fig.  20)  slides  along  the  beam  of  the 
machine,  its  position  at  the  moment  of  breaking  indicating 
the  tensile  strength  of  the  test  piece. 

Great  care  should  be  taken  to  place  the  test  piece  properly 
in  the  jaws  of  the  machine,  with  the  contact  between  the  two 
evenly  distributed.  After  each  piece  has  been  broken  it 
should  be  examined  to  ensure  that  no  side  strain  or  irregular 
pressure  has  been  applied.  Unless  very  great  care  is  taken 
when  placing  the  test  piece  in  the  machine  one  or  more  results 
will  be  much  below  the  average  ;  this  is  frequently  due  to  a 
defectively  shaped  test  piece,  or  to  its  not  having  been  placed 
truly  in  the  machine. 

(h)  The  rate  at  which  the  load  is  applied  must  be  the  same  in 
all  tests.  D.  B.  Butler  tested  over  800  samples  at  different 
rates  of  applying  the  load,  and  found  that  a  rapid  application 
greatly  increased  the  apparent  strength.  He  and  Faija 
therefore  adopted  as  a  standard  a  uniform  increase  in  load  of 
100  Ibs.  in  fifteen  seconds,  which  was  largely  adopted,  but  was 
afterwards  altered  to  100  Ibs.  in  twelve  seconds  when  the 
British  Standard  Specification  was  drawn  up. 

The  minimum  permissible  tensile  strength  of  test  pieces 
made  of  neat  cement  is  defined  by  the  British  Standard 
Specification  as  follows  :— 

"  The  average  breaking  strength  of  the  briquettes  seven 
days  after  gauging  must  not  be  less  than  400  Ibs.  per  square 
inch  of  section. 

"  The  average  breaking  stress  of  the  briquettes  twenty-eight 
days  after  gauging  must  show  an  increase  on  the  breaking  stress 
at  seven  days  after  gauging  of  not  less  than — 

25  per  cent,  when  the  seven-day  test  is  above  400  Ibs.  and  not  above  450  Ibs, 
20    .,       „         „        „    450  ,.       „     500  „ 
15    „       „         „        „    500  „       „     550  „ 
10    „       „         „        „    550  „       .,     600  „ 
5  600  , 


138      TESTING  THE  PROPERTIES  OF  CEMENTS 

(i)  The  age  of  the  cement  and  the  amount  of  aeration  it  has 
undergone  will  affect  the  tensile  strength.  Modern  cements  are 
not,  as  a  rule,  improved  by  aerating,  but  rather  the  contrary. 
No  really  satisfactory  explanation  for  this  has  been  offered. 

(j)  The  conclusions  drawn  from  different  tests  may  be  inaccurate. 
Thus,  it  is  customary  to  average  the  results  of  a  number  of 
tests  of  the  strength  of  cements  and  mortars,  but  the  accuracy 
of  such  an  average  is  seldom  great.  Cement  manufacturers 
consider  that  the  highest  result  of  a  series  represents  the  value 
of  a  cement  most  accurately,  and  attribute  the  lower  results 
to  discrepancies  in  the  gauging,  moulding  or  fitting  into  the 
testing  machine. 

CEMENT-SAND  TESTS. 

The  tensile  strength  of  neat  cement  is  a  purely  arbitrary 
factor,  as  cement  alone  is  never  used  to  withstand  heavy 
structural  strains,  but  for  this  purpose  is  always  mixed  with 
several  times  its  weight  of  inert  materials.  Indeed,  tests  of 
the  tensile  strength  of  neat  cement  will  frequently  lead  to 
erroneous  conclusions  in  the  absence  of  corresponding  tests 
on  mixtures  of  cement  and  sand.  Thus,  a  very  finely-ground 
cement  if  tested  neat  will  appear  to  be  inferior  to  a  coarser 
sample,  because,  in  the  latter,  the  coarser  particles  will  behave 
more  like  an  inert  material.  Mixtures  of  cement  and  sand, 
on  the  contrary,  show  the  great  advantage  to  be  derived  from 
fine  grinding. 

The  futility  of  tensile  tests  on  neat  cement  is  shown  by  the 
fact  that,  if  a  mixture  of  cement  with  an  equal  weight  of 
sand  is  finely  ground,  the  product,  when  tested  neat,  has  the 
same  tensile  strength  as  pure  Portland  cement.  With  the 
modern  demand  for  a  very  finely-ground  cement,  therefore,  in 
place  of  neat  tests  an  admixture  of  sand  should  be  employed 
in  estimating  the  value  of  a  cement. 

In  the  "  neat"  tensile  test  the  full  value  of  the  cement  as  a 
concreting  material,  i.e.  its  cementing  power,  never  comes  into 
play  ;  and  though  it  is  generally  recognised  that  a  finer  cement 
is  a  more  valuable  product,  yet  a  coarse  sample  in  a  neat  test 
will  give  results  as  to  tensile  strength  equal  to  those  obtained 
by  a  fine  cement.  On  the  other  hand,  the  difference  between 


TESTING  TENSILE  STRENGTH  139 

the  constructive  values  of  a  coarse  and  of  a  fine  cement  will  be 
most  noticeable  in  a  test  for  tensile  strength  if  carried  out  with 
a  mixture  of  sand  and  cement  in  the  proportion  of  3  :  1. 

The  value  of  sand  tests  in  the  place  of  neat  tests  is  becoming 
more  appreciated  day  by  day,  and  tensile  tests  on  neat  cement 
are  not  recognised  in  the  German,  Austrian  and  Swiss  Standard 
Specifications. 

In  order  to  obtain  concordant  and  comparable  results  it  is 
necessary  to  employ  a  "  standard  sand  ";  that  recognised  in  the 
British  Standard  Specification  being  obtained  from  Leighton 
Buzzard.  This  sand  is  "  thoroughly  washed,  dried  and  passed 
through  a  sieve  of  20  X  20  meshes  per  square  inch,  and  must 
be  retained  on  a  sieve  of  30  X  30  meshes  per  square  inch,  the 
wires  of  the  sieves  being  -0164  inch  and  -0108  inch  in  diameter, 
respectively."  The  German  standard  sand  is  rather  coarser,  as 
it  passes  through  a  plate  perforated  with  circular  holes  -054  inch 
diameter,  but  not  through  holes  -031  inch  diameter.  The 
French,  Austrian,  Swiss  and  American  standard  sands  are  of 
the  same  fineness  as  the  British.  The  Russian  sand  is  rather 
finer.  There  is  an  objection  to  the  use  of  a  different  material 
in  the  tests  to  that  which  will  be  employed  in  the  actual 
structure,  namely,  the  differences  between  the  two  materials 
will  prevent  a  true  comparison  of  the  results.  It  is,  indeed, 
possible  that  a  cement  will  be  condemned  when  tested  in 
admixture  with  standard  sand,  but  will  prove  highly  satis- 
factory .when  tested  as  a  portion  of  an  actual  structure.  This 
discrepancy  cannot,  at  present,  be  avoided  without  great 
difficulty  ;  substituting  the  sand  and  aggregate  to  be  used  in 
the  actual  structure  does  not  solve  the  problem,  as  low  tests 
on  a  given  cement  might  then  be  attributed  to  unsuitable 
aggregates  rather  than  to  defective  cement.  At  the  same  time 
the  discrepancy  between  cement-sand  mixtures  and  concrete 
made  from  the  same  cement  is,  in  some  cases,  so  serious  as  to 
make  tests  on  the  actual  concrete  imperative  in  all  important 
structures  where  failure  of  the  material  might  involve  loss  of 
life. 

The  tensile  strength  of  mixtures  of  cement  and  sand  is  very 
valuable  in  detecting  adulteration  in  the  form  of  inert  material, 
such  as  Kentish  rag  (limestone)  added  to  cement  to  cheapen 


140      TESTING  THE  PROPERTIES  OF  CEMENTS 

it.  So  far  as  the  test  on  neat  cement  is  concerned,  the  addition 
of  an  inert  material  will  frequently  increase  the  tensile  strength, 
but  the  admixture  will  readily  be  detected  by  the  low  results 
obtained  when  the  sand-cement  mixture  is  tested. 

The  great  drawbacks  to  testing  mixtures  of  sand  and  cement 
are  (a)  the  irregularities  in  the  results  unless  skilled  men  are 
employed,  and  (b)  the  length  of  time  required  for  the  test. 
The  latter  difficulty  is  not  appreciable  when  the  tests  described 
in  the  British  Standard  Specification  are  used,  as  the  time 
required  for  the  tensile  test  of  the  neat  cement  is  made  the 
same  as  that  for  the  cement-sand  mixture,  viz.,  twenty-eight 
days. 

The  variations  in  the  results  obtained  by  different  testers, 
or  even  by  the  same  man  on  different  occasions,  are  much 
greater  than  in  the  case  of  neat  cement  unless  great  manipulative 
skill  is  used  in  the  gauging  and  filling  of  the  moulds.  With 
skilled  operators  the  differences  are  not  important.  They  are 
due  almost  entirely  to  the  greater  difficulty  in  gauging  a 
cement-sand  mixture  and  in  handling  it  afterwards. 

The  following  extract  from  the  British  Standard  Specifica- 
tion represents  the  best  modern  practice  : — 

'  The  cement  shall  be  tested  by  submitting  to  a  tensile 
stress  briquettes  prepared  from  one  part  by  weight  of  cement 
to  three  parts  by  weight  of  dry  standard  sand,  the  said 
briquettes  being  of  the  shape  described  for  the  neat  cement 
tests. 

"  The  mixture  of  cement  and  sand  shall  be  gauged  with  so 
much  water  as  to  be  moist  throughout,  but  no  surplus  of  water 
shall  appear  when  the  mixture  is  gently  beaten  with  a  trowel 
into  the  mould.  Clean  appliances  shall  be  used  for  gauging, 
and  the  temperature  of  the  water  and  that  of  the  test  room  at 
the  time  the  said  operations  are  performed  shall  be  from 
58°  to  64°  F.,  and  no  ingredient  other  than  cement,  sand,  and 
clean,  fresh  water  shall  be  introduced  in  making  the  test.  The 
mixture  gauged  as  above,  shall  be  filled,  without  mechanical 
ramming,  into  moulds  of  the  form  shown  in  Fig.  18  (p.  133), 
each  mould  resting  upon  a  non-porous  plate  until  the  mixture 
has  set.  When  the  mixture  has  set  sufficiently  to  enable  the 
mould  to  be  removed  without  injury  to  the  briquettes,  such 


TESTING  TENSILE  STRENGTH  141 

removal  is  to  be  effected.  Each  said  briquette  shall  be  kept  in 
a  damp  atmosphere  for  twenty-four  hours  after  gauging,  when 
it  shall  be  placed  in  clean,  fresh  water  and  allowed  to  remain 
there  until  required  for  breaking,  the  water  in  which  the  test 
briquettes  are  submerged  being  renewed  every  seven  days, 
and  the  temperature  thereof  maintained  between  58°  and  64°  F. 

"  The  briquettes  shall  be  tested  for  breaking  at  seven  and 
twenty-eight  days  after  gauging,  six  briquettes  for  each  period. 
The  average  tensile  stress  of  the  six  briquettes  shall  be  taken 
as  the  tensile  stress  for  each  period.  For  breaking,  the 
briquette  shall  be  held  in  strong  metal  jaws,  of  the  shape 
shown  in  Fig.  19  (p.  134),  the  briquettes  being  slightly  greased 
where  gripped  by  the  jaws.  The  load  must  be  steadily  and 
uniformly  applied,  starting  from  zero,  increasing  at  the  rate 
of  100  Ibs.  in  twelve  seconds. 

"  The  average  breaking  stress  of  the  cement  and  sand 
briquettes  seven  days  after  gauging  must  be  not  less  than 
150  Ibs.  per  square  inch  of  section. 

"  The  average  breaking  stress  of  the  briquettes  twenty- 
eight  days  after  gauging  must  not  be  less  than  250  Ibs.  per 
square  inch  of  section,  and  the  increase  in  breaking  stress  from 
seven  to  twenty-eight  days  must  not  be  less  than  : — 

25  per  cent,  when  the  seven-dav  test  is  above  200  Ibs.  and  not  above  250  Ibs. 
15        „  „  „  „         250    „  „  300    „ 

10        „  „  „  „         300    „  „  350    „ 

°  55  55  55  55  350        ,, 

All  the  best  British  Portland  cements  give  much  higher 
results  than  those  just  mentioned,  and  a  large  number  tested 
under  the  author's  supervision  yielded  results  with  an  average 
of:- 

After  seven  days.  After  twenty-eight  days. 

Neat  cement,  660  Ibs.  per  square  inch.  800  Ibs.  per  square  inch 

Cement-Sand  {  2-ft  «-0 

1-3         I  "  "  "  " 

The  German  standard  is  confined  to  1:3  cement-sand 
mixtures,  which  must  have  a  tensile  strength  exceeding 
228  Ibs.  per  square  inch  after  twenty-eight  days,  and  a  com- 
pressive  strength  of  2,280  Ibs.  per  square  inch. 

The  French  specification  requires  the  gauging  to  be  done 


142      TESTING  THE  PROPERTIES  OF  CEMENTS 

with  sea-water,  and  gives  the  minimum  tensile  strength  as 
114  Ibs.  per  square  inch  after  seven  days,  and  214  Ibs.  per 
square  inch  after  twenty-eight  days,  with  a  further  proviso 
that  the  increase  in  strength  between  seven  and  twenty-eight 
days  must  be  at  least  24 J  Ibs.  per  square  inch.  A  further 
clause  in  the  French  specification,  which  is  not  always  insisted 
upon,  requires  the  strength  of  the  cement-sand  test  pieces  to 
be  at  least  256  Ibs.  per  square  inch  at  the  end  of  eighty-four 
days,  and  in  any  case  greater  than  at  the  end  of  twenty-eight 
days. 

Although  mechanical  ramming  is  not  allowed  in  the  above 
specification,  it  is  employed  on  the  Continent  and  (for  their 
own  satisfaction)  by  a  few  firms  in  Great  Britain.  Massive 
iron  moulds  must  then  be  used  in  which  a  piston  is  fitted,  and 
a  considerable  number  of  blows — usually  150 — are  then 
delivered  to  the  top  of  this  piston  at  the  rate  of  one  blow  per 
second,  either  by  means  of  a  small  tilt-hammer  devised  by 
Boehme,  or  by  a  miniature  pile-driving  rammer  designed  by 
Klebe.  The  latter  is  prescribed  in  the  Austrian  and  Swiss 
specifications.  Presses — of  either  the  "  screw  "  or  "  hydraulic  " 
type — have  been  used  to  consolidate  the  cement-sand  mixture 
in  the  moulds,  but  they  give  lower  results  than  do  mechanical 
hammers,  and  have  not  been  adopted  in  any  official  specifica- 
tions. 

The  tensile  strength  of  slag  cements,  mortars,  hydraulic 
lime,  pozzolanas,  etc.,  is  tested  in  a  manner  similar  to  that 
just  described,  though  slight  modifications  are  necessary  on 
account  of  the  different  nature  of  the  materials.  Slag  cement 
should  give  tensile  strength  results  almost  identical  with  those 
of  Portland  cement.  Feeble  hydraulic  limes  have  a  tensile 
strength  of  70  to  100  Ibs.  per  square  inch,  but  the  stronger 
limes  show  results  up  to  140  Ibs.  per  square  inch.  Roman 
cements,  rock  cements  and  natural  cements  vary  greatly,  but 
should  not  have  a  tensile  strength  below  130  Ibs.  per  square 
inch  ;  very  few  of  them  reach  as  high  as  200  Ibs.  per  square 
inch,  which  may  be  regarded  as  the  minimum  for  a  1  :  3 
Portland  cement-sand  mixture.  Pozzolanas  (including  trass) 
must  be  mixed  with  lime  as  well  as  sand  before  being  tested. 
According  to  M.  Gary,  a  mixture  of  equal  volumes  of  trass, 


TESTING  RESISTANCE  TO  BENDING 


143 


standard  lime  paste  and  standard  sand  should  have  a  tensile 
strength  of  at  least  200  Ibs.  per  square  inch.  The  standard 
lime  paste  used  is  made  by  mixing  slaked  or  hydrated  lime 
with  an  equal  weight  of  water. 


TRANSVERSE  BENDING  STRENGTH. 

In  most  cements  the  bending  strength  is  fairly  proportionate 
to  the  compressive  and  tensile  strengths,  and,  as  it  is  not  difficult 
to  measure,  its  determination  is  becoming  increasingly  popular, 
especially  as  the  variations 
between  different  tests  are  less 
than  in  those  of  tensile  and 
compressive  strengths. 

Though  not  yet  recognised 
in  any  official  specifications, 
the  resistance  of  cements  and 
cement  -  sand  mixtures  to 
bending  stresses  have  been 
extensively  studied.  Instead 
of  the  test  pieces  previously 
described,  prisms  (usually 
16  x  4  x  4  cm.)  are  employed, 
the  cement  and  sand  mixture 
being  gauged  with  one-tenth 
of  its  weight  of  water.  The 
prisms  are  tested  by  three 
knife  edges — two  below  and 
one  above  the  prism — which 


FIG.  22. — Schiile's  Machine  for 
Transverse  Bending  Tests. 


are  fitted  to  the  jaws  in  a  tensile  strength  testing  machine 
(Fig.  22),  the  load  being  applied  in  the  same  manner  as  when 
testing  tensile  strength. 

0.  Frey  has  found  that  the  bending  strength  of  a  number  of 
German  standard  cement-sand  mixtures  tested  by  him  is 
about  one-fifth  of  the  compressive  strength  or  double  the 
tensile  strength,  but  the  British  cements  examined  by  the 
author  show  lower  figures.  The  conclusions  which  may  be 
drawn  as  to  the  value  of  a  bending  test  may  be  summarised 
in  the  following  manner  :— 


144      TESTING  THE  PROPERTIES  OF  CEMENTS 

1.  The  testing  of  cements  by  means  of  prisms  made  from 
plastic  mortar  is  distinguished  by  its  extreme  simplicit}^  and 
by  the  great  advantage  that  both  bending  and  compression 
tests  can   be   obtained  from  the   same  block-samples.     This 
testing  process  is  the  solution  of  the  question  as  to  equal 
capacity  of  the  tensile  and  compression  test  samples. 

2.  The  individual  differences  compared  with  the  mean  are 
smaller  by  this  method  than  by  the  use  of  tensile  test  samples 
of  8  shape.     For  compression  tests  there  is  no  considerable 
difference  between  the  individual  deviations  and  the  mean 
values  given  by  the  results  with  cubes  or  with  prisms. 

3.  The  lower  figures  given  by  the  prism  method  correspond 
better  with  the  strengths  obtained  in  practice  than  do  the 
high  figures  obtained  with  material  of  earth-moist  consistency 
subjected  to  hard  ramming. 

4.  The  difficulty  of  exactly  determining  the  mixing- water  and 
the  work  of  ramming  is  solved  in  a  satisfactory  manner  by 
using  the  same  weights  of  the  cement,  sand  and  water  in  every 
test. 

5.  The  use  of  prisms  gives  the  compression  tests  the  import- 
ance which  they  deserve,  which  far  exceeds  that  of  the  tensile 
and    bending    strengths,    both    in    regularity    and    practical 
importance. 

SHEARING  STRENGTH. 

The  resistance  of  blocks  made  of  cement  and  sand  to  a 
shearing  stress  would  furnish  valuable  information  if  only  it 
could  be  measured  easily  and  accurately.  At  present,  however, 
this  is  not  the  case,  but  as  this  subject  is  receiving  a  considerable 
amount  of  attention,  a  satisfactory  method  may  shortly  be 
discovered.  Meanwhile,  bending  tests  (p.  143)  are  the  nearest 
approximation. 

OTHER  TESTS. 

A  number  of  other  properties  of  cements  and  cement-sand 
mixtures  have  been  proposed  as  the  basis  of  valuations,  but 
have  never  been  extensively  used. 

The  adhesion  of  cement  to  metal  is  an  important  property  as, 
in  reinforced  concrete,  it  tends  to  counteract  the  sliding  of  the 


VARIOUS  TESTS  145 

metallic  bars  embedded  in  the  material.  This  adhesion  or 
resistance  to  gliding  is  a  resultant  of  several  forces,  including 
the  resistance  of  the  cement  to  shearing,  the  pressure  exercised 
between  the  metal  and  the  cement,  the  alterations  in  volume 
accompanying  the  hardening  of  the  cement  and  the  action  of 
external  loads.  Unfortunately,  it  is,  at  present,  impossible  to 
measure  this  adhesion  accurately. 

Adhesiveness  is  tested  by  placing  a  mould  on  the  surface 
to  which  it  is  desired  the  cement  shall  adhere  (stone,  brick,  etc.), 
and  then  filling  the  cement,  cement-sand  mixture  or  mortar  as 
usual.  When  the  material  is  sufficiently  hard  the  mould  is 
removed  and  the  test  piece  is  subjected  to  tensile  strain  in  the 
ordinary  testing  machine.  The  most  satisfactory  results  are 
obtained  when  the  test  piece  has  the  shape  and  dimensions 
shown  in  Fig.  17.  Tests  of  adhesion  are,  usually,  tests  of  the 
"  other  "  material  rather  than  of  the  cement,  and  are  not 
required  except  in  very  unusual  circumstances. 

The  compactness  or  apparent  density  of  a  test  piece  is  deter- 
mined by  means  of  a  volumenometer,1  in  which  the  volume 
of  the  test  piece  is  measured.  The  volume  in  c.c.,  divided  by 
the  weight  of  the  test  piece  in  grammes,  is  the  apparent  density, 
and  may  be  regarded  as  a  measure  of  the  compactness. 

The  porosity  of  a  test  piece  is  determined  by  weighing  it, 
then  immersing  it  in  a  vessel  from  which  all  the  air  can  be 
exhausted.  Enough  water  must  then  be  admitted  to 
completely  cover  the  test  piece.  The  air  is  exhausted  so  as  to 
cause  the  water  to  enter  all  the  open  pores  in  the  test  piece ; 
the  latter  is  then  wiped  dry  and  then  re- weighed.  The 
increase  in  weight  will  be  the  water  absorbed  by  the  pores  in 
the  test  piece. 

The  porosity  of  test  pieces  made  of  neat  cement  is  extremely 
low  ;  that  of  concrete  aggregates  is  much  higher  and  is 
preferably  determined,  as  described  in  the  section  on  concrete. 

1  For  a  description  of  this  apparatus,  see  the  author's  "  British  Clays,  Shales  and 
Sands,"  C.  Griffin  &  Co.,  London,  1911. 


CHAPTER  VI 

THE    COMPONENTS    OF    CONCRETE    AND    THEIR    PROPERTIES 

CONCRETE  may  be  defined  as  an  inert  material,  the  particles 
of  which  are  united  by  cement  to  form  a  hard,  stony  mass 
useful  for  all  the  ordinary  purposes  of  natural  building  stones. 

The  use  of  concrete  has  been  extended  enormously  during 
the  past  few  years,  and  now  covers  a  wide  sphere  of  usefulness 
in  the  construction  of  bridges,  breakwaters,  docks,  canals, 
dams,  reservoirs,  paving  stones,  partitions,  roofing  tiles, 
building  blocks,  boats,  rafts,  conduits,  water  mains,  sewers, 
telegraph  poles,  fences,  caissons,  pipes,  etc.,  etc.  Each  year 
new  uses  are  made  of  this  remarkable  material,  and  our 
knowledge  of  its  properties  is  increased. 

The  inert  material  is  usually  composed  of  sand  of  different 
degrees  of  fineness  and  a  coarser  material  termed  an  aggregate. 
Other  materials  are,  however,  introduced  in  special  cases  to 
confer  additional  properties  on  the  concrete.  Thus,  pozzolanas 
(p.  161)  are  sometimes  added  to  increase  the  hardness  of  the 
mass,  whilst  very  finely  ground  rock,  dust  or  clay  is  sometimes 
used  to  secure  a  denser  concrete.  Clay  is  inadvisable  for  this 
purpose  on  account  of  its  plasticity  and  water-repelling  power. 

The  components  of  concrete  are  usually  four  in  number  : 
(a)  cement,  (b)  sand,  (c)  aggregate,  and  (d)  water.  To  these 
must  be  added  metal  (usually  steel)  in  the  case  of  reinforced 
concrete. 

The  apparent  simplicity  of  concrete  has  led  to  its  abuse  in 
many  ways,  and  there  is  widespread  belief  that  any  inert 
material  and  almost  any  kind  of  lime  or  cement  may  be  used 
with  satisfactory  results.  This  is  altogether  wrong,  for  in 
reality  the  production  of  a  satisfactory  concrete  is  by  no  means 
simple,  and  involves  considerable  knowledge,  continual  super- 
vision and  conscientious  work. 


COMPONENTS  OF  CONCRETE  147 

CEMENT. 

Many  kinds  of  cementitious  material  may  be  employed  for 
concrete,  the  strength  of  the  structure  depending  on  that  of 
the  cement  and  of  the  aggregate.  Lime  and  hydraulic  lime 
concretes  are  largely  used  for  foundation  work  on  account  of 
the  low  cost ;  natural  cements  (p.  29)  are  used  for  somewhat 
stronger  work,  and  Portland  cement  (p.  20)  where  the  best  and 
strongest  concrete  is  required,  and  for  all  reinforced  concrete. 

The  strength  and  reliability  of  Portland  cement  are  such 
that  it  is  rapidly  replacing  other  kinds  in  all  the  most  important 
work.  Care  should,  however,  be  taken  that  only  Portland 
cement  of  good  quality,  and  preferably  that  complying  with 
the  requirements  of  the  British  Standard  Specification  (p.  97) 
should  be  used.  For  greater  accuracy  of  working  as  well  as 
convenience,  the  cement  should  be  delivered  in  bags  or  barrels 
containing  a  definite  weight  of  cement.  This  weight  and  the 
maker's  name  should  be  marked  legibly  on  the  bags  or  barrels. 

WATER, 

The  water  ordinarily  available  is  suitable,  unless  it  contains 
a  notable  quantity  of  humic  acid  or  other  organic  matter  which 
retards  the  setting  of  the  cement.  The  water  used  should 
always  be  clean  ;  that  from  ponds  and  streams  is  liable  to 
contain  clay  and  other  detrimental  matter  in  suspension. 
In  case  of  doubt  it  should  be  run  into  a  settling  tank  or  filtered 
through  sand.  The  water  should  therefore  be  tested  to  ascer- 
tain the  effect  (if  any)  of  its  constituents  on  the  rate  at  which 
the  cement  sets.  Hard  water  should  be  avoided  wherever 
possible  as  the  action  of  the  salts  dissolved  in  it  is  always 
uncertain  and  often  detrimental.  Carefully  collected  rain 
water  is  the  most  generally  suitable  kind  of  water ;  that  from 
wells  sunk  in  the  chalk  is  the  most  unsatisfactory  of  all  kinds 
of  fresh  water.  Sewage  and  other  effluents  should  never 
be  used. 

AGGREGATES. 

Many  varieties  of  stone,  broken  bricks,  coke,  clinker,  ashes 
and  other  substances  of  a  stony  character  may  be  used  as 

L  2 


148  COMPONENTS  OF  CONCRETE  AND  PROPERTIES 

aggregates  in  concrete,  the  selection  of  any  particular  piece 
of  work  depending  on  the  purpose  for  which  the  concrete  is 
to  be  used,  the  strength  it  is  desired  the  concrete  should  possess, 
the  accessibility  and  cost  of  each  aggregate-material  and— 
in  many  instances — the  whim  of  the  architect  or  engineer  in 
charge  of  the  work.  Strictly  speaking,  the  whole  of  the  solid 
non-cementitious  material  used  in  concrete  constitutes  the 
aggregate,  but  this  term  is  usually  applied  solely  to  the  coarser 
portion,  the  finer  being  termed  "  sand." 

Aggregates  used  in  concrete  may  be  divided  into  four 
groups  :  (a)  natural  stones,  such  as  granite,  limestone,  etc.  ; 

(b)  artificial  stones,  such  as  burned  clay,  broken  bricks,  etc.  ; 

(c)  by-products  of  various  kinds,  such  as  coke,  blast-furnace 
slag,  clinker  or  ashes ;  and  (d)  pozzolanas  or  trass  (p.  159). 

Natural  stones  must  usually  be  broken  into  pieces  of  con- 
venient size,  but  those  which  occur  in  the  form  of  gravel  have 
the  advantage  that,  if  free  from  sand  and  clay,  they  are  ready 
for  use  without  any  further  preparation.  The  most  suitable 
are  angular  fragments  of  moderate  density  ;  rounded  pebbles 
from  the  sea-shore  or  river  beds  (ballast)  are  less  satisfactory, 
but  are  used  in  large  quantities  on  account  of  their  convenience. 
Fortunately,  the  difference  between  the  useful  strength  of 
concrete  made  with  pebbles  and  that  made  with  angular 
pieces  is  not  sufficient  to  be  of  importance  in  most  instances. 

What  are  known  to  geologists  as  igneous  rocks  furnish 
many  excellent  aggregates,  though  the  basalts,  traps,  felsites 
and  denser  lavas  have  too  high  a  specific  gravity  (2-9  to  3-2) 
to  be  desirable.  The  lighter  lavas,  such  as  pumice  stone,  on 
the  contrary,  have  so  low  a  crushing  strength  that  they  can 
only  be  used  for  partitions  and  other  work  where  lightness 
rather  than  strength  is  required.  Where  pumice  stone  is  not 
available,  a  good,  light  concrete  of  a  similar  nature  may  be 
made  from  hard  coke.1  Some  of  the  basalts  have  the  further 
disadvantage  of  a  glossy  surface,  to  which  the  cement  does  not 
adhere  satisfactorily. 

Granite,  if  washed  free  from  adventitious  substances  and 
powder,  is  a  very  suitable  aggregate  for  almost  any  class  of 

1  The  risk  of  fire  must  not  be  overlooked.  This  danger  is  often  exaggerated 
(see  p.  150). 


NATURE  OF  AGGREGATES  149 

structure.  Granite-concrete  is  dense  and  rather  heavy  (i.e.,  of 
rather  high  specific  gravity),  but  is  particularly  useful  for  large 
structures  and  for  maritime  work.  It  is  costly,  except  in  those 
localities  where  granite  occurs,  but  is  universally  recognised  as 
the  best  aggregate. 

As  the  presence  of  fine  adherent  granite  dust  is  detrimental, 
the  granite  chippings  should  be  thoroughly  washed  before  use. 
In  quarries  when  fans  or  dust  extractors  are  used  the  granite 
is  more  free  from  dust,  and  is  to  that  extent  superior — in  the 
absence  of  facilities  for  washing  the  material. 

Limestones  are  amongst  the  most  valuable  sedimentary 
rocks  which  may  be  used  as  aggregates,  and  are  so  widely 
distributed  as  to  be  generally  available.  The  harder  varieties 
with  a  specific  gravity  of  2-7  are  the  most  suitable  for  the 
purpose.  The  most  artistic  results  are  obtained  with  the 
particular  variety  of  limestone  known  as  Portland  stone,  on 
account  of  the  close  resemblance  it  bears  to  Portland  cement, 
thereby  enabling  the  concrete  to  be  carved  and  dressed  in  a 
manner  impossible  when  an  aggregate  of  a  different  colour  is 
used. 

The  chief  disadvantage  of  limestone  as  an  aggregate  is  the 
decomposition  it  undergoes  when  subjected  to  the  action  of 
fire,  resulting  in  the  shrinkage  and  rapid  collapse  of  the 
structure. 

Sandstones,  quartzites  and  other  siliceous  stones  are  generally 
very  suitable  as  aggregates,  but  shales,  slates  and  micaceous 
sandstones  should  be  avoided,  as  the  flat  fragments  they 
produce  do  not  form  a  strong  concrete. 

In  some  districts — particularly  in  the  eastern  counties — 
flint  pebbles  are  the  only  stones  available.  They  make  an 
excellent  concrete  for  use  at  ordinary  temperatures,  but  one 
which  is  apt  to  "  fly  "  and  split  when  heated.  This  tendency 
is  reduced  by  crushing  the  flints  in  a  stonebreaker,  but  their 
great  hardness  makes  this  costly. 

The  artificial  stones  used  as  aggregates  consist  chiefly  of 
broken  bricks,  terra-cotta  or  burnt  clay  ballast.  Inferior 
material  should  be  avoided,  as  it  has  a  low  crushing  strength 
and  is  very  liable  to  crumble.  The  materials  selected  should 
be  hard  and  preferably  should  show  signs  of  vitrification. 


150  COMPONENTS  OF  CONCRETE  AND  PROPERTIES 

Soft  and  crumbly  material  should  be  avoided.  In  addition 
to  their  primary  nature  as  hard  and  inert  materials,  all  products 
composed  of  burnt  clay  have  cementitious  properties  when 
mixed  with  lime  (see  pozzolanas,  p.  35),  and  so  tend  to  increase 
the  strength  of  the  concrete.  This  is  particularly  the  case 
where  the  material  is  ground  to  the  state  of  sand  (p.  159). 

If  broken  bricks  are  used  they  should  be  carefully  cleaned 
from  any  adherent  mortar  and  dust.  Broken  pottery,  tiles 
and  pipes  are  unsuitable,  as  the  pieces  "  bridge  over  "  each 
other  and  form  excessively  large  voids. 

Bricks  from  some  localities  are  occasionally  found  to  cause 
"  blowing  "  or  cracking  in  the  concrete,  but  the  reason  for  this 
has  never  been  satisfactorily  explained.  The  statement  some- 
times made  that  Fletton  bricks  contain  an  excessive  amount 
of  sulphur  does  not  apply  to  most  of  the  bricks  from  the 
Fletton  district,  though  it  is  an  unquestionable  fact  that,  in 
several  instances,  concrete  made  of  bricks  alleged  to  have 
been  made  near  Fletton  has  proved  unstable.  Whether  its 
failure  be  due  to  the  method  employed  in  making  the  concrete 
or  to  the  broken  bricks  used  is,  at  present,  impossible  to  say. 

Coke-breeze,  when  sufficiently  resistant  to  crushing,  forms 
a  good  aggregate,  but  the  softer  coke  which  is  sometimes 
substituted  is  dangerous.  Coke  has  been  chiefly  used  as  an 
aggregate  in  concrete  floors  and  walls,  as  it  makes  a  light 
construction  into  which  nails  may  be  easily  and  securely 
driven.  It  has  somewhat  undeservedly  fallen  into  disrepute 
because  of  its  prohibition  in  Germany  where  its  quality  is 
distinctly  inferior  to  that  of  good  English  coke. 

It  has  also  been  suggested  that  coke  forms  an  inflammable 
aggregate,  but  the  tests  made  by  the  Fire  Prevention  Committee 
with  various  kinds  of  floors  exposed  for  three  hours  to  a  rapidly 
increasing  and  ultimate  temperature  of  1,900°  F.  followed  by 
a  spray  of  cold  water,  placed  coke  breeze  and  burnt  clay  ballast 
as  first,  and  showed  them  to  be  (after  the  tests)  quite  free 
from  cracks  and  deflection,  and  much  superior  to  granite  so 
far  as  these  particular  tests  were  concerned. 

The  fear  of  expansion  and  disintegration  of  coke-concrete 
in  ordinary  use  also  appears  to  be  largely  imaginary.  The 
chief  disadvantage  of  coke  is  the  presence  of  oxidisable  sulphur 


NATURE  OF  AGGREGATES  151 

compounds  in  it  ;  these  expand  and  rupture  the  concrete, 
sometimes  to  an  alarming  extent.  What  is  known  as  "  steel 
coke  "  is  usually  very  free  from  sulphur  compounds,  and  may 
be  highly  recommended  where  a  light  aggregate  is  desired,  but 
coke  rich  in  sulphur  is  undesirable,  and  ashes  are  a  particularly 
unsuitable  material  to  mix  with  the  coke.  Coke  breeze  should 
not  be  used  for  reinforced  concrete. 

Ashes  are  similar  to  coke,  but  usually  contain  a  large  pro- 
portion of  sulphur  and  so  are  undesirable,  though  much  used 
on  account  of  the  low  cost.  Ashes  from  locomotive  boilers 
are  considered  to  be  superior  to  domestic  ashes,  the  steam  in 
the  forced  draught  being  supposed  to  effect  a  volatilisation  of 
the  sulphur  present  in  the  coal.  Care  should  be  taken  that 
the  ashes  are  free  from  admixture  with  hydraulic  lime,  as  lias 
limestone  is  mixed  with  some  of  the  coal  used  by  engine 
drivers. 

Clinker  and  furnace  slag  are  stronger  aggregates  than  coke  or 
ashes,  but  suffer  from  the  presence  of  sulphur  compounds  which, 
when  oxidised,  expand  and  may  cause  the  destruction  of  the  con- 
crete. Provided  that  the  proportion  of  sulphur  is  sufficiently 
small  to  be  practically  harmless,  and  that  the  clinker  is  hard  and 
free  from  dust,  shale,  lime,  ash,  metal  scrap  or  scale  and  basic 
slag,  slags  form  valuable  aggregate  material,  their  general 
availability  and  low  cost  rendering  them  very  attractive  to 
builders  and  contractors.  The  fact  should  not  be  overlooked, 
however,  that  many  slag  heaps  vary  very  greatly  in  com- 
position, and  that  tests  do  not  necessarily  represent  the  percen- 
tage of  sulphur  in  any  and  every  part  of  the  heap.  Hence, 
there  is  always  some  risk  in  using  slag  unless  its  composition 
is  exceptionally  uniform.  The  commercial  attractiveness  of 
slags  as  aggregates  must  not  be  allowed  to  obscure  the  danger 
of  collapse  which  accompanies  their  use,  except  when  the 
construction  of  a  building  has  been  under  the  most  strict 
supervision. 

Clinker  and  slags  should  not  be  used  for  reinforced  concrete, 
except  under  protest. 

If  the  particles  composing  the  aggregate  are  of  various  sizes 
the  mass  will  be  more  compact  than  if  all  the  particles  are 
large  and  uniform  in  size.  Thus  a  mixture  of  broken  stone. 


152  COMPONENTS  OF  CONCRETE  AND  PROPERTIES 


cement  and  sand  will  occupy  a  smaller  volume  than  when  each 
of  the  materials  are  kept  separately.  This  is  clearly  shown  in 
Figs.  23 — 26.  In  Fig.  24  is  shown  a  plan  of  a  cubical  box 
containing  twenty-seven  spheres  of  equal  size  ;  there  is  a 
considerable  amount  of  space  between  each  of  these  spheres, 
even  though  the  box  is  apparently  "  full."  In  Fig.  25  the 
same  box  is  shown  with  a  number  of  smaller  spheres  packed 


J    L 


Gemsnt  Sa.nd  Stone  Concrete 

FIG.  23. —  Space  occupied  by  Concrete  and  its  components. 

in  between  the  larger  ones,  so  that  the  box  now  contains 
considerably  more  material  than  it  did  before.  At  the  same 
time  it  is  clear  that  there  is  still  space  enough  to  contain  a 
comparatively  large  quantity  of  sand  and  fine  powder  before 
all  the  spaces  or  voids  between  the  particles  would  be  com- 
pletely filled.  If  instead  of  truly  spherical  particles,  irregularly 
shaped  ones  are  used,  the  same  facts  will  be  observed,  and  this 


FIG.  24. 


FIG.  25. 


FIG.  26. 


is  what  actually  occurs  in  the -preparation  of  concrete  (Fig.  26). 
From  this  it  is  clear  that  most  aggregates  must  be  graded  or 
separated  into  fragments  of  suitable  sizes,  as  if  this  is  not  done 
either  a  weak  concrete  will  be  obtained  or  a  very  large  amount 
of  cement  will  be  wasted.  As  the  cost  of  the  additional  cement 
is  far  greater  than  the  expense  of  grading  the  aggregate,  a 
careful  grading  of  the  material  effects  a  considerable  saving 


NATURE  OF  AGGREGATES  153 

in  cost.  It  is  possible  to  make  a  difference  of  nearly  100  per 
cent,  in  the  strength  of  a  concrete  by  careful  attention  to  the 
grading.  If  the  aggregate  is  composed  of  particles  all  of  one 
size,  the  amount  of  voids  will  be  about  50  per  cent.  ;  if  it  is 
composed  of  two  sizes  of  particles,  the  proportion  of  voids 
will  shrink  to  between  30  and  40  per  cent.  ;  by  still  more 
variation  in  the  size  of  the  particles,  it  is  possible  to  get  the 
amount  of  voids  down  to  20  to  25  per  cent.,  and  in  a  very 
weak  proportion,  such  as  1  :  15,  there  is  no  reason  why  one 
should  not  include  an  amount  of  fine  sand  so  as  to  reduce  the 
percentage  of  voids  still  lower.  To  use  a  lot  of  sand  will  not 
always  so  reduce  the  percentage  of  voids,  because  sand,  which  is 
composed  of  a  fairly  regular  form  of  grain,  will  likewise  contain 
50  per  cent,  of  voids.  A  gradation  of  size  is  necessary  in  either 
large  or  small  material  in  order  to  reduce  the  amount  of  voids, 
it  being  the  gradation  in  size  which  enables  the  particles  to 
arrange  themselves  more  closely  together  and  leave  a  smaller 
proportion  of  interstices.  It  is  easy  to  see  that  in  a  properly 
graded  material  wherein  the  voids  are  a  minimum,  the  cement 
will  go  much  further  and  the  concrete  will  be  denser  and 
stronger.  The  principle  applies  equally  to  a  poor  concrete, 
such  as  1:5:1,  or  a  rich  mixture,  such  as  1:1:2  (see 
p.  167). 

Many  gravels  require  no  grading,  as  the  particles  they 
contain  are  all  of  sizes  and  in  proportion  suitable  for  use  as 
aggregates  ;  this  is  one  of  the  advantages  of  such  gravels. 
Other  gravels  and  most  of  the  larger  materials  must  first  be 
crushed  to  fragments  of  a  suitable  size,  the  crushed  product 
being  then  divided  by  sieves  into  particles  of  certain  pre- 
arranged sizes. 

The  size  of  the  largest  particles  permissible  in  an  aggregate 
depends  to  some  extent  on  the  size  of  the  mass  of  concrete  to 
be  produced.  For  most  purposes  the  aggregate  should  pass 
completely  through  a  hole  one  inch  diameter,  though  for 
very  large  blocks  some  of  it  may  be  large  enough  to  just  pass 
through  a  3-inch  hole.  The  Second  Report  of  the  Com- 
mittee of  the  Royal  Institute  of  British  Architects  states  that 
I  inch  is  the  usual  maximum  size  allowable.  In  very  large 
masses  of  concrete,  such  as  are  used  in  maritime  work,  large 


154     COMPONENTS  OF  CONCRETE  AND  PROPERTIES 

blocks  of  stone  are  embedded  at  intervals.     The  usual  practice 
is  to  screen  through  sieves  of  the  meshes  as  under  : — 

For  Artificial  -paving  blocks  .       J  inch  to  f  inch. 
Floors  .          ,  .       1     „      „   |     „ 

Walls   .          .          ;  '       .1       „      „  2  inches. 
Foundations  .          .     2|  inches. 

"  Plums  "  or  large  stones  used  in  ordinary  foundation  work 
should  be  carefully  deposited  well  away  from  one  another,  and 
from  the  sides  and  angles  of  the  mass  ;  they  are  never  allowable 
in  reinforced  work. 

All  material  which  will  pass  through  a  hole  J-inch  diameter 
should  be  removed  by  sifting  or  washing  or  a  combination  of 
these  processes,  as  the  presence  of  loam  and  clay  is  highly 
detrimental,  and  sand  of  unusual  fineness  has  been  known  to 
cause  a  failure  in  the  concrete.  An  aggregate  which  contains 
more  than  3  per  cent,  of  clay  should,  generally  speaking,  be 
rejected. 

It  is  sometimes  urged  that  the  removal  of  sand  from 
aggregates  is  undesirable,  especially  as  some  sand  must  be 
added  to  the  concrete  mixture  in  order  to  fill  the  voids  in  the 
coarser  aggregate.  For  this  reason,  some  engineers  wash  the 
aggregate  so  as  to  remove  earth,  dust,  clay  and  other  very 
fine  particles,  but  endeavour  not  to  remove  the  coarser  sand. 
If  the  aggregate  is  of  a  very  uniform  character  this  method  may 
be  adopted  with  success,  but  in  most  instances  it  will  be  found 
that  the  distribution  of  the  sand  in  the  aggregate  is  so  irregular 
that  it  is  almost  impossible  to  ascertain  accurately  the  propor- 
tion present.  The  removal  of  all  particles  which  will  pass 
through  a  hole  J-inch  diameter  secures  an  aggregate  of  much 
greater  uniformity  as  regards  the  voids  and  greatly  assists 
in  the  production  of  a  concrete  of  maximum  strength. 

Whether  a  particular  aggregate  requires  washing  may  be 
ascertained  in  a  rough,  but  usually  sufficient  manner  by  a 
method  recommended  by  the  Associated  Portland  Cement 
Manufacturers,  Limited.  A  tall  glass  cylinder,  graduated  in 
c.c.  and  of  at  least  50  c.c.  capacity,  is  half  filled  with  aggregate 
and  water  is  then  added  to  fill  the  cylinder  about  three-quarters 
full.  The  cylinder  is  then  closed  with  a  cork  or  rubber  stopper. 


NATURE  OF  AGGREGATES 


155 


and  it  is  then  shaken  violently  so  as  to  wash  out  all  clay  and 
sand.  The  cylinder  is  then  placed  on  a  bench  and  its  contents 
are  allowed  to  settle.  After  a  few  minutes  the  sand  and  most 
of  the  clay  will  have  settled  on  top  of  the  aggregate,  and  its 
proportion  to  the  latter  can  be  roughly  gauged  by  comparing 
the  respective  volumes  of  each.  The  apparatus  required  is 
shown  in  Fig.  27. 

It  should  be  observed  that  the  foregoing  method  does  not 
show  the  presence  of  sulphurous  slag,  coke  breeze  and  certain 
other  deleterious  ingredients  of  a  coarse  nature.  If  these  are 
suspected,  the 
aggregate  must  be 
subjected  to  a  more 
searching  investiga- 
tion. 

The  removal  of 
the  fine  material  is 
best  effected  by 
washing  in  a  shallow 
trough,  the  bottom 
of  which  is  per- 
forated. The  output 
is  increased  if  the 

trough  is  vibrated  PlG.  27,— Apparatus  for  determining  whether 
in  a  manner  similar  aggregates  require  washing  before  use. 

to  the   jiggers  used  (By  courtesy  of  the  Associated  Portland  Cement 

,  .  ,        ,  Manufacturers,  Ltd. ) 

tor  washing  coal  and 

ores.  The  method — frequently  adopted — of  throwing  the 
material  against  a  steeply  inclined  riddle  is  not  satisfactory,  and 
fails  to  remove  a  large  proportion  of  the  finer  material.  Where 
no  jigger  is  available,  metal  wheelbarrows,  the  sides  and  bottom 
of  which  are  perforated  with  J-inch  holes,  may  be  substituted. 
The  concrete  aggregate  is  delivered  in  piles  near  the  work,  and 
carried  from  them  to  the  mixer  in  wheelbarrows.  Between 
the  mixer  and  the  storage  piles  a  water-pipe  is  connected  up, 
with  a  large  perforated  nozzle  at  its  discharge  end.  Each 
wheelbarrow  load  of  gravel  on  its  way  to  the  mixer  is  rolled 
under  the  nozzle  and  streams  of  water  discharged  upon  it,  the 
material  being  churned  about  with  a  spade  to  expose  the 


156  COMPONENTS  OF  CONCRETE  AND  PROPERTIES 

lower  part  of  the  load  to  the  cleansing  action  of  the  water.  The 
water  and  the  loam  pass  out  through  the  perforations  in  the 
wheelbarrow,  and  the  clean  gravel  is  then  carried  to  the  mixer 
and  used. 

Aggregates  should  always  be  wetted  thoroughly  before  being 
used,  and  the  additional  labour  of  screening  is  so  insignificant 
in  relation  to  the  advantages  gained  that  it  should  never  be 
omitted.  This  is  particularly  the  case  where  the  aggregate  is 
composed  of  crushed  limestone,  the  fine  dust  of  which  is 
peculiarly  detrimental  and  may  reduce  the  ultimate  strength 
of  the  concrete  by  as  much  as  40  per  cent. 

It  is  a  curious  fact  that  the  larger  the  proportion  of  fine 
aggregate  the  weaker  will  be  the  concrete.  Thus,  a  concrete 
made  of  small  pieces  of  sandstone  and  cement  will  be  much 
stronger  than  one  made  of  the  same  materials,  but  with  the 
stone  reduced  to  powder.  Consequently,  it  is  desirable  to  use 
an  aggregate  of  a  coarse  nature  and  also  with  as  few  voids 
as  possible.  In  order  to  ensure  this,  the  aggregate  should 
all  be  passed  through  a  screen  with  holes  of  a  prearranged 
size  (say  1J  inch  diameter),  then  washed  and  passed  over 
another  screen  with  holes  J  inch  diameter. 

As  suitably  graded  aggregates  yield  stronger  concretes  than 
those  in  which  the  fragments  of  aggregate  are  all  the  same 
size,  the  Testing  Committee  of  the  Concrete  Institute  have 
suggested  that  the  aggregate  should  "  be  sifted  to  the  following 
degrees  and  the  percentage  of  voids  ascertained  of  (1)  the 
whole,  and  (2)  of  each  separate  grading  :— 

To  pass  an  aperture  of —  To  be  retained  on  an  aperture  of — 

£  inch  by  f  inch  f  inch  by  f  inch 

~z        »         h     »  i*        "         's      " 


No  definite  proportions  of  either  size  of  particle  or  of  voids  have 
yet  been  specified.  It  is,  however,  recognised  that  the  more 
varied  the  sizes  the  denser  and  stronger  will  be  the  concrete, 
and  the  smaller  the  proportions  of  cement  and  sand  needed. 

The  proportion  of  each  grading  to  the  whole  and  the  specific 
gravity  of  the  aggregate  should  also  be  ascertained.  The 
aggregate  thus  obtained  should  then  be  tested  to  ascertain  the 


MEASURING  THE  PROPORTION  OF  VOIDS      157 

proportion  of  voids  present.     This  should  not  exceed  45  per 
cent,  and  will  seldom  be  less  than  25  per  cent. 

The  percentage  of  voids  in  an  aggregate  or  sand  is  deter- 
mined in  a  manner  .similar  to  the  following,  various  small 
refinements  being  possible  where  a  more  accurate  determina- 
tion is  required:  The  aggregate  to  be  tested  is  well  wetted 
and  allowed  to  drain  on  a  sloping  sheet  of  glass  so  as  to 
remove  all  surplus  water  from  the  surface  without  withdrawing 
any  from  the  pores.  Two  precisely  similar  graduated  glass 
cylinders,  each  of  about  1,000  c.c.  capacity,  are  then  placed 
ready  for  use.  In  the  first  cylinder  is  placed  a  convenient 
quantity  of  the  aggregate.  This  is  shaken  and  tamped  down 
so  as  to  make  it  as  compact  as  possible  without  breaking  the 
fragments.  The  volume  of  the  aggregate  in  the  cylinder  is 
then  carefully  noted;  it  should  be  about  600  c.c.,  but  must 
be  measured  as  exactly  as  possible.  In  the  second  cylinder 
is  placed  400  c.c.  of  water.  The  measured  quantity  of 
aggregate  is  then  completely  transferred  to  the  second  cylinder, 
and  any  air  bubbles  in  the  latter  are  removed  by  probing 
carefully  with  a  wire.  The  contents  of  the  second  cylinder 
are  then  shaken  so  that  no  air  spaces  exist  in  the  aggregate, 
and  after  a  few  moments  settling  the  height  of  the  water  level 
in  this  cylinder  is  noted  by  means  of  the  graduations  on  the 
glass.  The  difference  between  the  volume  of  aggregate  and 
water  before  mixing  and  after  mixing  will  be  the  volume  of 
the  voids  in  the  quantity  of  aggregate  tested,  and  from  this 
the  percentage  of  voids  may  be  easily  calculated.  Thus, 
supposing  that  the  volume  of  aggregate  used  was  640  c.c.,  the 
volume  of  water  400  c.c.  (making  a  total  volume  before  mixing 
of  1,040  c.c.)y  and  that  the  volume  in  the  second  cylinder 
(after  mixing)  was  787  c.c.,  then  the  difference  in  volume  on 
mixing  (due  to  the  voids)  would  be  253  c.c.,  and  the  percentage 
of  voids  will  be — 


253  X  100 

— — =  39*5  per  cent. 

640 


The  apparatus  used  is  shown  in  Fig.  28  by  courtesy  of  the 
Associated  Portland  Cement  Manufacturers,  Ltd. 

If  the  aggregate  is  porous  (like  coke)  it  must  be  well-soaked 


158    COMPONENTS  OF  CONCRETE  AND  PROPERTIES 


in  water  before  being  put  into  the  measuring  vessel,  as  it  is 
the  space  between  the  particles  and  not  the  porosity  of  the 
particles  themselves  which  it  is  desired  to  measure.  Materials 
of  a  highly  porous  nature,  yet  with  somewhat  large  pores,  are 
troublesome,  as  each  piece  must  be  immersed  in  paraffin  wax 
before  being  tested,  for  unless  the  pores  are  sealed  in  this  way 
the  water  will  run  out  of  them  in  transferring  the  aggregate 
to  the  measuring  vessel  and  an  erroneous  result  will  be  obtained. 
Fortunately,  in  the  majority  of  materials  used  as  aggregates, 
the  porosity  is  so  small  that  its  influence  may  be  neglected  in 
making  the  test,  provided  the  aggregate  is  well  wetted. 

The  proportion  of  voids  is  important  in  two  ways  :  first,  it 

serves  as  an  index 
of  the  suitability 
of  the  aggregate  as 
regards  the  size  of 
the  particles,  and 
secondly,  it  serves 
as  a  measure  of  the 
amount  of  sand 
which  must  be 
added  to  the  con- 
crete, as  will  be 
described  later. 
The  sand  used  in 

concrete    consists 
FIG.  28.— Apparatus  for  determining  proportion  essentjanv      of      an 
of  voids  in  aggregates.  J 

inert   material,    the 

particles  of  which  are  sufficiently  small  to  occupy  the 
spaces  or  voids  between  the  larger  fragments  of  aggregate. 
The  composition  of  the  sand  is  relatively  unimportant 
providing  that  it  is  free  from  clay.  The  latter  is  usually 
removed  by  washing  the  sand  in  a  stream  of  water  either  in 
large  tanks  or,  preferably,  in  a  rotating  drum  ;  the  clay, 
roots,  grass,  seeds,  etc.,  are  carried  away  by  the  water  and 
the  clean  sand  remains  behind.  Washing  sand  does  not 
always  improve  it,  however,  as  in  some  cases  the  finest 
particles  thus  removed  are  of  value. 

Most  contractors  are  willing  to  pay  a  good  price  for  cement, 


MEASURING  THE  PROPORTION  OF  VOIDS       159 

but  are  anxious  to  use  the  cheapest  sand  available.  This  is 
a  mistake,  as  the  final  strength  of  the  concrete  will  be  seriously 
reduced  if  an  unsuitable  sand  is  employed. 

Sands  may  be  divided  into  three  groups  :  (a)  siliceous, 
(b)  calcareous,  and  (c)  pozzolanic. 

The  siliceous  sands  are  by  far  the  most  widely  used,  and 
consist  of  the  material  abraded  by  the  action  of  wind,  weather 
and  water  from  the  siliceous  rocks.  The  purest  siliceous  sands 
consist  almost  exclusively  of  fragments  of  quartz  and,  under 
the  microscope,  may  be  seen  to  be  composed  of  small,  clear 
glassy  particles  with  either  sharp  or  rounded  edges  according  to 
the  conditions  to  which  they  have  been  subjected.  River  sand, 
the  particles  of  which  are  sharper  and  more  angular  than 
those  of  sea  sand,  is  preferable  for  use  in  concrete. 

The  general  properties  of  siliceous  sands  are  well  known. 
Those  chiefly  used  in  concrete  are  :— 

(a)  Pit  sand  (other  than  that  of  glacial  origin). 

(6)  River  sand. 

(c)  Sea  sand. 

(d)  Grit  or  sand  from  crushed  coarse  material.     This  may  be 
of  a  mixed  nature  and  contain  calcareous  matter. 

Calcareous  sands  do  not  occur  naturally  in  a  sufficiently 
pure  state  to  be  used  in  concrete.  They  are  prepared  by 
crushing  limestone  rocks  to  powder  and  screening  out  the 
coarser  particles.  It  has  been  found  that  calcareous  sands 
yield  stronger  yet  lighter  concretes  than  do  purely  siliceous 
ones,  but  the  much  lower  cost  of  the  latter  makes  it  only 
natural  that  siliceous  sands  should  chiefly  be  employed. 

Pozzolanic  sands  are  made  by  crushing  burnt  clay  products, 
such  as  broken  bricks,  terra-cotta,  or  clay  which  has  been 
calcined  specially  or  by  grinding  natural  pozzolanas  or  trass 
(p.  35)  to  a  fine  powder.  These  sands  have  the  advantage  of 
forming  cements  in  the  presence  of  the  free  lime  produced  by 
the  hydrolysis  of  the  cement,  and  thereby  forming  a  stronger 
concrete  than  when  entirely  inert  sand  is  used.  This  fact  was 
well  known  to  the  ancient  Romans,  who  mixed  ground  pot- 
sherds with  their  mortar  to  increase  its  strength. 

The  great  disadvantage  of  Portland  cement  is  the  formation 
of  free  lime  when  the  cement  sets.  This  free  lime  is  gradually 


160     COMPONENTS  OF  CONCRETE  AND  PROPERTIES 

washed  out  of  the  structure — especially  in  maritime  works — 
and  leaves  a  porous  mass  which  is  liable  to  corrosion  and 
decay  in  proportion  to  its  porosity.  Indeed,  there  are  few 
works  of  importance,  in  which  Portland  cement  has  been 
used,  which  do  not  show  the  serious  results  of  this  loss  of  lime. 

In  order  to  prevent  these  defects,  some  material  must  be 
added  which  will  combine  with  the  lime  set  free  by  the 
hydration  of  the  cement.  The  ideal  substance  for  this  pur- 
pose is  a  trass  or  pozzolana,  as  these  unite  with  lime  to  form 
a  new  cement.  The  proportion  of  trass  to  be  added  should 
be  slightly  in  excess  of  that  required  to  neutralise  the  lime  set 
free  in  the  cement.  This  is,  theoretically,  rather  more  than 
one-quarter  of  the  Portland  cement.  Hence,  the  best  propor- 
tion of  trass  is  half  that  of  the  Portland  cement  in  the 
concrete  mixture.  This  trass  may  legitimately  take  the  place 
of  part  of  the  sand  used  in  making  the  concrete. 

Concrete  composed  of  1  measure  of  cement,  J  measure  of 
trass,  and  5  measures  of  (sand  -f  aggregate)  has  proved 
particularly  durable  and  resistant  to  sea  water.  It  is  not 
improbable,  in  fact,  that  the  simultaneous  use  of  both 
Portland  cement  and  trass  may  go  far  towards  solving  the 
problems  raised  by  the  action  of  sea  water  on  concrete. 

The  use  of  pozzolanic  sands  in  concrete  has  not  been  extensive 
in  this  country,  notwithstanding  their  obvious  advantages.1 

The  shape  and  hardness  of  the  sand  grains  is  often  more 
important  than  their  composition.  Flat  grains,  derived  from 
micaceous  stones  or  shales,  are  liable  to  form  a  weak  concrete 
on  account  of  their  shape.  More  spherical  grains  with  sharp 
angular  projections  are  the  most  suitable. 

The  size  of  the  sand  particles  should  not  be  too  small.  The 
British  specification  for  Standard  Sand  limits  the  size  of  the 
grains  to  those  which  will  pass  through  a  No.  20  sieve,  but  will 
be  retained  on  a  No.  30  sieve  (p.  139).  The  Standing  Committee 
of  the  Concrete  Institute  fix  the  upper  limit  of  sand  as  that 
which  passes  through  a  J-inch  by  J-inch  aperture,  and  the 
lower  limit  as  that  which  is  retained  on  a  ^-inch  by  ^-inch 
aperture.  The  Second  Report  of  the  Committee  of  the  Royal 

1  Further  information  on  this  important  subject  will  be  found  in  "  A  Manual  for 
Masons,"  by  J.  A.  van  der  Kloes  and  A.  B.  Searle.  (London:  J.  &  A.  Churchill.) 


SIZE  OF  SAND  GRAINS  161 

Institute  of  British  Architects  specify  a  "  sand  composed  of 
hard  grains  of  various  sizes  up  to  particles  which  will  pass  a 
|-inch  square  mesh,  but  of  which  at  least  75  per  cent,  should 
pass  a  J-inch  square  mesh.  Fine  sand  alone  is  not  suitable." 
The  Report  also  states  that  "  the  value  of  a  sand  cannot  always 
be  judged  from  its  fineness,  and  tests  of  the  mortar  prepared 
with  the  cement  and  the  proposed  sand  should  always  be 
made." 

As  coarse  sand  has  fewer  voids  than  fine  sand,  it  is  always 
preferable,  and,  in  addition  to  yielding  a  stronger  material, 
it  requires  less  cement. 

Graded  sands  give  better  results  than  those  in  which  all  the 
grains  are  of  approximately  the  same  size,  as  in  well-graded 
sands  there  are  fewer  voids.  For  this  reason  the  Tests  Com- 
mittee of  the  Concrete  Institute  have  suggested  that  the 
"  sand  "  shall  be  sifted  to  the  following  degrees,  and  the 
percentage  of  voids  ascertained  of  (1)  the  whole,  and  (2)  of 
each  separate  grading  :— 

To  pass  an  aperture  of —          To  be  retained  on  an  aperture  of — 
J  inch  by  |  inch  £  inch  by  |  inch 

8"  »  ¥        >»  TS  »          16       » 

Tff         »  Tff     »  T51-!  »        aV     » 

sV         »  3*5     »  sV  »>        ihf     »> 

The  proportion  of  each  grading  to  the  whole  and  the  specific 
gravity  of  the  sand  should  also  be  ascertained.  No  definite 
proportions  have  been  fixed. 

The  proportion  of  voids  in  sand  varies  with  the  size  of  the 
grains.  In  sand  suitable  for  concrete  it  varies  from  23  to  40 
per  cent.  The  voids  are  determined  in  precisely  the  same 
manner  as  those  in  the  aggregate  (p.  157). 

The  tensile  strength  of  test  pieces  made  of  standard  cement 
and  the  proposed  sand  should  be  ascertained,  so  as  to  ensure 
that  the  concrete  is  not  weakened  by  the  use  of  unsuitable 
sand.  The  best  method  of  determining  the  tensile  strength 
is  that  described  on  p.  138,  but  substituting  the  sand  to  be 
tested  for  the  Leighton  Buzzard  sand  there  mentioned. 


c.  M 


CHAPTER  VII 

THE  PREPARATION  OF  CONCRETE 

THE  manufacture  of  a  concrete  article  and  the  erection  of 
a  structure  of  concrete  is  similar  to  the  making  of  a  casting 
in  a  foundry.  Forms  or  patterns  are  built  to  correspond 
exactly  with  the  shape  of  the  finished  work,  the  reinforcing 
steel  (if  any)  is  set  in  place,  and  the  concrete  is  poured  into  the 
forms.  The  whole  structure  is  thus  built  or  moulded  into  the 
finished  form  as  a  single  piece  or  monolith.1  The  concrete  is 
allowed  to  set  a  requisite  length  of  time,  the  forms  are  removed, 
and  the  building  or  article  stands  complete — a  structure 
carved,  as  it  were,  out  of  solid  rock. 

Hence,  the  work  of  construction  or  erection  consists  primarily 
of  four  distinct  operations  :  (1)  the  erection  of  the  forms, 
centering  or  false  work  ;  (2)  the  placing  of  the  reinforcing 
steel  (in  reinforced  concrete)  ;  (3)  the  mixing  and  pouring  or 
placing  of  the  concrete  within  the  forms  ;  and  (4)  the  removal 
of  the  forms  or  centering. 

As  the  forms  represent  the  mould  from  which  the  finished 
structure  is  made,  great  care  is  used  to  make  these  exact  and 
true  to  line.  They  must  be  built  rigid  and  thoroughly  braced 
so  as  to  bear  the  weight  of  the  plastic  concrete  without  deflec- 
tion. In  order  to  give  a  smooth  finish,  surface-planed  boards 
are  used,  and  the  corners  of  all  columns  and  beam  boxes  must 
be  chamfered.  All  joints  should  be  set  closely  together  so  as 
to  make  the  forms  fairly  water-tight. 

The  steel  is  set  accurately  in  place  in  accordance  with 
detailed  drawings  prepared  for  the  purpose  by  the  architect, 
engineer  or  firm  of  concrete  specialists,  and  these  drawings 
should  be  followed  explicitly. 

In  the  production  of  concrete  from  its  various  components — 
aggregates,  sand,  cement  and  water — it  is  of  the  greatest 

1  From  monos  =  one,  or  a  single,  lithos  —  a  stone  or  rock. 


PROPORTIONS  OF  INGREDIENTS  163 

importance  that  every  available  means  should  be  adopted  to 
secure  a  product  of  the  highest  quality.  To  this  end  the 
greatest  care  and  skill  should  be  used  in  the  selection  of  the 
materials,  in  mixing  them  thoroughly  in  the  most  suitable 
proportions  and  in  applying  the  concrete  thus  produced  with 
all  necessary  speed.  It  is  equally  important  that  the  climatic 
and  other  conditions  should  be  suitable,  for  the  production  of 
concrete  in  times  of  frost  is  always  detrimental  to  its  quality 
and  strength,  though  if  certain  precautions,  such  as  those 
described  on  pp.  187,  et  seq.,  are  taken,  the  loss  of  strength  may 
be  unimportant. 

The  results  of  carelessness,  ignorance  or  lack  of  skill  in  the 
production  and  shaping  of  concrete  are  so  serious  and  are 
occasionally  attended  with  so  great  a  loss  of  life  that  no  con- 
demnation of  them  can  be  too  strong,  and  no  precautions  to 
prevent  their  occurrence  can  be  too  severe. 

PROPORTIONS. 

The  proportions  in  which  the  various  components  of  concrete 
should  be  mixed  together  must  be  ascertained  separately  for 
the  aggregate  used  in  each  case.  The  strength  and  cost  of 
the  concrete  will  depend  largely  on  the  correctness  or  otherwise 
of  these  proportions,  yet  it  is  surprising  how  little  attention  is 
paid  to  this  matter  by  many  users.  Just  because  some 
builders  have  found  that  for  certain  aggregates  used  by  them 
a  given  proportion  of  sand  and  cement  gave  the  best  results, 
numerous  others  have  concluded  that  for  other  aggregates  the 
same  proportions  are  the  most  suitable. 

It  is  generally  agreed  that  the  strongest  concrete  is  that 
which  contains  the  smallest  proportion  of  voids,  and  the 
primary  object  of  grading  the  sand  and  aggregate  is  to  secure 
this.  At  the  same  time,  the  proportion  of  cement  present 
must  be  sufficient  to  coat  every  particle  of  material  in  the 
concrete,  and  in  this  way  to  secure  the  firm  adhesion  of  all 
the  particles  to  each  other.  The  builder  or  engineer  who  uses 
concrete  has  to  avoid,  on  the  one  hand,  the  production  of  too 
weak  a  material  by  the  use  of  too  little  cement  and,  on  the 
other,  the  useless  employment  of  cement  (which  is  expensive) 

M  2 


164     THE  PREPARATION  OF  CONCRETE 

merely  to  fill  the  voids  or  spaces  between  the  particles.  It  is 
sometimes  stated  that  only  those  particles  of  the  cement  which 
are  left  over  when  the  voids  have  all  been  filled  can  be  used  as 
cementitious  material,  but  this  is  not  necessarily  the  case,  as 
a  strong  concrete  may  sometimes  be  obtained  notwithstanding 
the  presence  in  it  of  a  large  percentage  of  voids. 

That  the  percentage  of  voids  cannot  in  any  way  be  judged 
by  inspection  is  clearly  shown  in  Figs.  29  and  30.  The 
limestone  shown  in  Fig.  29  had  37-5  per  cent,  of  voids,  and 
consisted  of  grains  of  the  following  sizes  :— 

Per  cent. 
Eetained  on  No.  10  sieve  44 


»  »          20       ,, 

30       „ 

40       „ 

50      „ 

Passed  thrqugh  a  No.  50  sieve 


21-4 
8-5 
4-0 
3-4 

18-7 


whilst  8  per  cent,  of  that  shown  in  Fig.  30  is  passed  through 
a  No.  50  sieve.  Yet  the  fine  stone  contained  40-5  per  cent, 
of  voids,  required  9  per  cent,  more  cement  than  the  coarser 
stone,  and  the  resultant  concrete  had  only  one-third  the 
tensile  strength  of  that  made  from  the  coarser,  but  better 
graded  stone. 

The  minimum  proportion  of  cement  is  that  which  will 
cover  the  whole  surface  of  each  particle  of  aggregate  and  of 
each  grain  of  sand  with  a  coating  of  sufficient  thickness  to 
cause  the  particles  of  aggregate  and  sand  to  adhere  so  firmly 
to  each  other  as  to  form  a  hard,  stone-like  mass.  This  mini- 
mum is  unattainable  in  practice,  as  it  would  involve  coating 
each  grain  and  particle  separately  with  cement,  and  making 
no  allowance  for  cement  filling  up  any  voids  in  the  material. 
In  practice  these  voids  are  seldom,  if  ever,  filled  completely 
with  cement,  and  it  has  been  found,  as  the  result  of 
innumerable  tests,  that  the  strongest  concrete  is  prepared  as 
follows  : — 

The  aggregate — carefully  selected,  and  with  particles  within 
the  limits  of  size  mentioned  on  pp.  154,  156 — is  tested  to 
ascertain  the  percentage  of  voids  in  it  (p.  157).  The  proportion 
of  voids  in  the  sand  must  also  be  ascertained.  The  quantity 
of  sand  (consisting  of  suitably  sized  grains,  as  described  on 


PROPORTIONS  OF  INGREDIENTS 


165 


-- 


p.  160)  to  be  added  to  this  aggregate  is  equal  in  volume  to 
the  total  volume  of  voids  in  the  aggregate  used.1 

The  quantity  of  the  cement  to  be  added  is  then  equal 
to  the  volume  of  the  voids  in  the  sand  used  plus  an  addi- 
tional quantity  to  effect  the  cementation  of  the  various  par- 
ticles. The  "  allowance  "  is 
usually  10  per  cent,  of  the 
cement  required  to  fill  the 
voids.  (Some  engineers 
prefer  15  per  cent.)  In  no 
case  should  the  amount 
of  cement  used  be  less 
than  will  fill  the  voids  in 
the  volume  of  sand  used 
in  the  concrete.  It  is 
generally  advisable  to  use 
a  larger  proportion,  as  the 
strength  of  the  concrete  is 
thereby  greatly  increased. 

Thus,   if    a  given  aggre- 
gate  has    40  per   cent,    of 
voids    and    the     sand 
be    used    with    it    has 


per  cent,  of  voids, 
each  100  measures 
aggregate  there  will 
required  .40  measures 


to 
33 
to 
of 
be 
of 


i      A   33 
sand  and  - 

100 


X  40  =  13-2  f 


13-2 
~LO~ 


FIG.  29. — Limsstoiie. 


=  14-5  measures  of  cement. 


In  other  words,  the  proportions  of  aggregate,  sand  and  cement 
will  be  100  :  40  :  14-5,  or  7  :  2f  :  1. 

It  must  be  observed  that  the  volume  of  cement  required  is 
not  that  of  the  dry  material  as  received  from  the  manufacturers, 
but  that  of  the  cement  after  it  has  set  and  hardened.  This 
must  be  determined  by  making  a  test  piece  of  neat  cement  and 
water,  using  an  accurately  weighed  quantity  of  cement,  and 
measuring  the  volume  of  the  piece  after  a  sufficient  length  of 


1  It  is  sometimes  preferable  to  replace  part  of  the  sand  by  finely  ground  trass 
or  pozsolana,  as  described  on  p.  159. 


166 


THE  PREPARATION  OF  CONCRETE 


time  (say  after  twenty-eight  days1).  It  will  then  be  possible 
to  calculate  the  volume  which  will  be  occupied  by  any  given 
weight  of  cement  under  the  conditions  in  which  it  exists  in 
concrete.  It  is  usual  to  reckon  1  cubic  foot  of  dry  cement 
powder  as  equivalent  to  0'85  cubic  foot  of  cement  in  actual 
use. 

Where  large  quantities  of  concrete  are  used  it  is  better  to 
reverse  the  calculations  and  to  start  with  a  bag  of  cement  as 
the  unit.  The  weight  of  Portland  cement  is  usually  marked 

clearly  on  the  bags  in 
which  it  is  sold,  so  that 
no  weighing  is  necessary. 
For  the  purposes  of  pro- 
portioning the  amount  of 
cement  to  be  added,  one 
cubic  foot  of  Portland 
cement  may  be  said  to 
weigh  90  Ibs.  Thus,  if 
an  aggregate  has  50  per 
cent,  of  voids  and  a  sand 
has  45  per  cent,  of  voids, 
the  proportions  of  aggre- 
gate and  sand  and  cement 
will  be  100  :  50  :  24-75, 
or  for  a  bag  containing 
100  Ibs.  of  cement  (=  1-111 
cubic  feet)  there  will  be 
required  4-5  cubic  feet  of 
aggregate  and  2*25  cubic 
feet  of  sand. 

The  advantage  of  speci- 
fying the  cement  by  weight  and  not  by  measurement  is  that 
the  makers  are  prepared  to  guarantee  the  weight  of  material 
contained  in  each  bag,  whereas  a  contractor  can  easily  make 
a  difference  of  5  to  10  per  cent,  in  the  weight  of  cement 
contained  in  a  given  volume. 

1  In  neat  cement  some  of  the  cement  acts  as  an  inert  material,  so  that  the  volume 
found  in  this  manner  is  less  than  that  which  would  be  occupied  by  cement  in 
concrete.  This  error  may  be  neglected,  as  it  is  on  the  safe  side. 


me 

FIG.  30. — Fine  Limestone. 


PROPORTIONS  OF  INGREDIENTS 


167 


Parts 

Parts          Parts 

Parts. 

Mortar. 

Cement.       Sand. 

Mortar 

1-20 

1      +      2        = 

2'35 

1'50 

1      +     2|      = 

2'70 

1*90 

14-3       = 

3'00 

The  joint  committee  formed  under  the  auspices  of  the  Royal 
Institute  of  British  Architects  in  1907  recommended  that 
the  proportions  of  cement  and  sand  should  be  settled  with 
reference  to  the  strength  of  concrete  required,  and  the  volume 
of  mortar  produced  by  the  admixture  of  the  sand  and  cement 
proposed  to  be  used  should  be  determined  in  each  case.  On 
small  works,  it  suggests,  the  following  figures  may  be  taken 
as  a  guide,  and  are  probably  approximately  correct  for  medium 
siliceous  sand  :— 

Parts  Parts 

Cement.        Sand. 

1       +         \      = 
1       +      1         = 

i+i!== 

These  proportions  are  only  correct  for  Portland  cement  ; 
when  Zirae-concretes  are  required  the  volume  of  sand  plus 
aggregate  must  never  be  greater  than  six  times  that  of  the 
cement.  The  much  greater  cementing  power  of  Portland 
cement  enables  strong  concretes  to  be  made  which  contain 
only  6|  per  cent,  of  cement. 

The  proportions  of  the  components  of  concrete  are  usually 
expressed  in  the  form  of  a  double  ratio  without  any  words. 
Thus,  a  1:2:4  mixture  is  understood  to  be  composed  of  one 
measure  of  cement,  two  measures  of  sand,  and  four  measures 
of  aggregate.  The  following  mixtures  are  in  common  use  :— 

(a)  In  a  rich  mixture  for  columns  and  other  structural  parts 
subjected   to   high   stresses   or   requiring   exceptional   water- 
tightness,  1  :  1J  :  3. 

(b)  In  a  standard  mixture  for  reinforced  floors,  beams,  and 
columns,  for  arches,  for  reinforced  engine  or  machine  founda- 
tions subject  to  vibrations,  for  tanks,  sewers,  conduits,  and 
other  water-tight  work,  the  proportions  should  be  1  :  2  :  4. 

(c)  A   medium  mixture  for  ordinary  machine  foundations, 
retaining    walls,    abutments,    piers,    thin    foundation    walls, 
building  walls,  ordinary  floors,  side  walks,  and  sewers  with 
heavy  walls  :    proportions,  1  :  2J  :  5. 

(d)  A  lean  mixture  for  unimportant  work  in  masses,   for 
heavy  walls,  for  large  foundations  supporting  a  stationary  load, 
and  for  backing  for  stone  masonry  :  proportions,    1:3:6, 


168 


THE  PREPARATION  OF  CONCRETE 


In  order  to  obtain  the  best  results  with  the  least  wastage  of 
cement,  however,  the  proportions  of  aggregate,  sand  and  cement 
should  be  calculated  in  the  manner  described  on  p.  164. 

To  measure  the  aggregate  and  sand,  boxes  of  suitable  sizes 
are  used.  Sometimes  a  barrel  with  both  ends  removed  is  used 
as  a  measure.  It  is  placed  on  the  platform  or  mixing  board 
and  is  filled  with  the  material.  The  barrel  is  then  lifted  off 
and  the  correct  measure  of  material  remains  on  the  board. 
It  is  incorrect  to  place  a  second,  smaller  barrel,  on  top  of  the 
first  one  and  to  fill  the  smaller  barrel  with  sand,  as  some  of  the 
sand  trickles  down  into  the  voids  of  the  coarser  aggregate  and 
wrong  proportions  are  obtained. 

It  is  convenient  to  make  the  measuring  boxes  of  such  a 
capacity  that  they  correspond  exactly  to  the  sand  and  aggregate 
needed  for  two  bags  of  cement,  or,  if  much  larger  quantities 
are  needed,  to  four  bags  of  cement.  Larger  quantities  than 
this  are  inadvisable  except  under  special  circumstances.  The 
following  table  shows  proportions  of  materials  and  the  sizes 
of  the  measuring  boxes  :— 

TABLE  SHOWING  THE  QUANTITIES  OF  MATERIALS  AND  THE 
RESULTING  AMOUNT  OF  CONCRETE  FOR  TWO-BAG  BATCH 
(EQUAL  TO  168}  LBS.  CEMENT)  (WILSON  AND  GAYLORD). 


Proportions 
by  parts. 

Two-bag  Batch. 

IE 

> 

Materials. 

Size  of  Measuring  Boxes. 
Inside  Measurements. 

III 

Big 

& 

C5 

*  8b£ 

Cement. 

TS 
1 

Stone  or 

Cement. 

«a 

1 

o_- 

II 

£o 

Concrete 

Sand. 

Stone  or  Gravel. 

x'iig 

ii  :« 

•5"o£  = 

Bags. 

Cu.  ft. 

Cu.  ft. 

Cu.  It. 

Gals. 

1 

2 

4 

2 

31 

H 

8A 

2'  x  2'  x  Hi" 

2'  X  4'  X  11|" 

10 

1 

3 

6 

2 

53 

ll\ 

12 

2'  x  3'  x  1H" 

3'  x  4'  x  ll|" 

is| 

For  measuring  water  a  tall,  narrow  bucket  or  cylinder  is 
the  most  suitable.  This  bucket  should  be  checked  as  to 
capacity  and  should  be  marked  to  distinguish  it  from  all 
others.  No  other  bucket  should  be  used  for  measuring. 


MEASURING  OUT  THE  INGREDIENTS 


169 


It  is  easy  to  make  a  mistake  in  proportioning  concrete 
mixtures  and  it  is,  therefore,  necessary  to  use  some  experi- 
mental method  in  order  to  ascertain  if  the  proportions  of  each 
ingredient  in  the  concrete  is  correct.  To  do  this  a  rough,  but 
usually  sufficiently  accurate  method  recommended  by  the 
Associated  Portland  Cement  Manufacturers,  Ltd.,  may  be 
used.  The  apparatus  required  is  shown  in  Fig.  31  to  consist 
of  a  large  and  small  measure  of  metal,  a  combined  funnel  and 
strainer  fitted  with  gauze  of  J-inch  mesh,  two  500  c.c.  graduated 
glasses  and  a  vessel  capable  of  delivering  a  fine  stream  of 
water.  This  last  is  used  for  washing  the  material. 

To  check  a  mixing  of  concrete 
a  sample  is  drawn  from  the  heap 
in  one  of  the  measures  (the  large 
one  being  used  if  concrete  is 
poor  in  cement,  or  contains  large 
aggregate).  The  strainer  is  held 
over  one  of  the  graduated  glasses 
and  the  sample  placed  therein 
(the  measure  being  washed  out 
to  ensure  inclusion  of  all  cement 
and  sand).  The  large  stones  are 
caught  on  the  mesh  of  the 
strainer,  and  the  cement  and 
sand  pass  through  into  the  glass, 
this  separation  being  assisted  by 
stirring  and  washing  down  with 
water  from  the  aspirator.  The  large  aggregate  left  in  the 
strainer  is  measured  in  the  second  glass  to  determine  whether 
it  is  approximately  correct.  The  agitation  of  the  cement  and 
sand  by  the  water  in  the  first  graduated  glass  will  cause  them 
to  separate  ;  the  sand  settling  more  rapidly  than  the  cement. 
After  standing  for  about  15  minutes,  the  approximate  pro- 
portions can  be  readily  seen  by  means  of  the  graduations. 
Slight  allowance  must  be  made  for  the  fact  that  the  cement 
will  not  settle  quite  so  compactly  as  the  sand. 

The  testing  of  two  samples  of  equal  quantity  from  different 
parts  of  the  heap  will  show  whether  the  mixing  is  uniform. 

This  test  will  not  show  minor  and  unimportant  variations 


FIG.  31.  —  Apparatus  for 
checking  the  composition 
of  Concrete  Mixtures. 


170     THE  PREPARATION  OF  CONCRETE 

from  the  specified  proportions,  but  its  use  will  prevent  such 
serious  trouble  as  occurred  recently,  when  after  a  new  sea-wall 
had  been  demolished  by  a  storm,  it  was  found  by  analysis  that 
the  concrete,  instead  of  being  4  :  1  as  specified,  was  approxi- 
mately 9:1. 

This  test  cannot  be  applied  to  concrete  which  has  set,  but 
only  to  freshly  made  mixtures. 

CONSISTENCY. 

The  amount  of  water  required  depends  on  the  consistency 
desired,  on  the  temperature  at  the  time  of  mixing  and  also 
on  the  materials  composing  the  aggregate.  Four  different 
consistencies  are  recognised  by  users  of  concrete  :— 

(1)  Grout  consisting  of  cement,  sand  and  water  in  the  form  of 
a  thick  cream  or  slurry.     Very  little  aggregate  can  be  present 
unless  its  particles  are  small. 

(2)  Very  wet  mixture,  consisting  of  concrete  wet  enough  to 
flow  off  a  shovel  yet  not  so  fluid  as  grout.     This  mixture  is 
largely  used  for  those  portions  of  reinforced  concrete  where 
the  metal  work  is  close  together. 

(3)  Dry    mixture    having    a    consistency  resembling    damp 
earth.     It  is  used  for  foundations  and  wherever  the  concrete  is 
required  to  set  rapidly.     This  mixture  must  be  thoroughly 
rammed  or  tamped  so  as  to  secure  a  uniform  distribution  of 
the  water  in  it.     It  should  be  observed  that  the  dry  mixture 
is  not  really  dry,  but  may  have  had  6  to  12  per  cent,  of  water 
added  to  it.     It  sets  with  inconvenient  rapidity,  and  owing  to 
the  tamping  needed  it  is  liable  to  contain  an  excessive  pro- 
portion of  voids  unless  worked  by  skilled  men. 

(4)  Medium,  or  ordinary  mixture,  in  which  the  material  is 
plastic  or  jelly-like.     To  remove  air  bubbles  and  fill  the  voids 
it  is  necessary  to  ram  or  tamp  this  mixture  lightly.     It  is 
used  for  all  the  ordinary  purposes  of  concrete.     It  has  the 
consistency  which  is  the  safest  for  men  of  average  skill.     25  to 
30  per  cent,  of  water  is  usually  necessary,  but  the  proportions 
vary  greatly.     One  large  firm  of  concrete  users  employ  one 
gallon  of  water  to  each  cubic  foot  of  dry  material  (=16  per 
cent.),  but  in  warm  weather  this  is  increased  to   1J  gallons 


CONSISTENCY  OF  CONCRETE  171 

(=  25  per  cent.).  Another  equally  important  firm  generally 
uses  20  per  cent,  of  the  volume  of  cement  plus  sand,  or  about 
10  per  cent,  of  the  whole  mixture. 

Notwithstanding  these  variations  in  the  consistency  of  the 
mixture,  it  is  generally  agreed  that  no  more  water  should 
be  added  than  is  necessary  to  effect  the  desired  chemical 
changes  in  the  cement,  plus  an  added  amount  just  sufficient 
to  enable  the  particles  to  slide  over  each  other  and  to  form  a 
plastic  mass.  As  some  aggregates  are  very  porous  they  should 
be  thoroughly  soaked,  and  the  surplus  water  drained  off  before 
adding  the  water  necessary  to  act  on  the  cement. 

The  quantity  of  water  to  be  used  must  be  found  by  tests,1 
but  it  is  generally  agreed  that  the  mass  should  be  sufficiently 
plastic  to  be  packed  easily  into  the  required  position,  and  yet 
should  not  be  so  wet  as  to  allow  any  dripping  of  the  cement, 
warter  or  sand.  This  point  is  shown  when,  after  ramming  the 
mass,  the  water  just  shows  on  the  surface. 

A  standing  committee  of  the  Concrete  Institute  has  suggested 
the  following  specification  for  the  consistency  of  concrete : — 

For  mass  concrete  the  quantity  of  water  added  to  the  other  con- 
stituents shall  be  sufficient  to  make  a  plastic  mixture  which,  after 
thorough  ramming,  will  quiver  like  a  jelly. 

For  reinforced  concrete  the  quantity  of  water  added  to  the  other 
constituents  shall  be  such  that  the  plastic  mixture  is  capable  of  being 
rammed  into  all  parts  of  the  moulds  and  between  the  bars  of  the 
reinforcement.  In  dry  weather  the  quantity  of  water  shall  be  increased 
in  order  to  allow  for  evaporation. 

Some  architects  and  engineers,  on  the  contrary,  stipulate 
that  the  mixture  shall  not  "  quake  "  or  quiver  like  a  jelly  when 
rammed.  This  is  to  avoid  the  production  of  a  "  mushy  "  mass, 
.such  as  is  generally  employed  in  the  United  States.  In  Great 
Britain  a  mass  of  so  sloppy  a  consistency  that  it  is  difficult  to 
keep  on  a  shovel  is  considered  unsuitable  and  liable  to  cause 
an  undue  segregation  of  the  cement  near  the  bottom  of  the 
material.  A  slight  excess  of  water  is  better  than  too  small  a 
quantity,  though  under  normal  conditions  the  best  results  are 
obtained  when  there  is  neither  an  excess  or  a  deficiency  of 
water  in  the  mixture.  A  concrete  which  has  been  worked  in 

1  See  "A  Manual  for  Masons,"  by  J.  A.  van  der  Kloes  and  A.  B  Searle 
(J.  &  A.  Churchill,  London). 


172     THE  PREPARATION  OF  CONCRETE 

too  dry  a  condition  does  not  pack  properly,  and  so  is  weaker 
than  one  in  which  more  water  has  been  used. 

The  difference  between  the  strength  of  dry  and  wet  mixtures 
in  the  early  stages  of  hardening  is  strongly  in  favour  of  the 
former,  but  after  several  months  the  strength  of  both  mixtures 
is  approximately  equal. 

There  are  a  few  rough-and-ready  methods  of  telling  whether 
the  consistency  of  concrete  is  right.  If  the  concrete  is  placed 
in  a  barrow,  by  the  time  it  is  wheeled  into  its  place  it  should 
not  have  taken  a  horizontal  surface.  A  shovelful  of  concrete 
from  the  bank  held  at  a  slight  angle,  should  show  no  signs  of 
the  cement  dropping  away.  Also  a  hole  made  in  the  concrete 
in  the  barrow  should  never  be  filled  in  by  any  ordinary  amount 
of  vibration. 

The  proportion  of  water  which  is  used  in  laboratory  tests  is 
usually  too  low  for  mixing  on  a  larger  scale.  In  the  tests,  the 
concrete  is  mixed  on  impervious  glass  plates  and  is  moulded 
in  non-porous  metal  moulds,  whilst  in  the  works  the  mixing 
board  and  the  moulds  or  centering  both  absorb  a  large  amount 
of  water,  the  exact  amount  of  which  it  is  almost  impossible 
to  ascertain,  as  it  varies  with  the  dryness  of  the  timber. 

The  nature  of  the  structure  has  some  importance  on  the 
proportion  of  water  which  is  most  suitable.  Thus,  in  beams — 
particularly  near  the  points  of  support — the  concrete  needs  to 
be  very  fluid  to  pass  between  the  closely-set  reinforcing  bars. 
In  plain  masses  of  concrete,  on  the  contrary,  a  much  drier 
mixture  is  preferable,  and  where  the  thickness  of  the  concrete 
is  very  great  (as  in  foundations)  the  last  portions  must  be 
almost  dry  to  counteract  the  effect  of  excess  of  water  in  the 
other  portion  which  is  brought  to  the  surface  in  ramming  or 
tamping. 

When  used  for  submarine  work,  as  in  docks,  harbours,  etc., 
the  mixture  must  appear  to  be  dry,  though  in  reality  it  contains 
a  considerable  proportion  (6—12%)  of  water.  Perfectly  dry 
mixtures  of  aggregate,  sand  and  cement  should  never  be 
employed. 

In  moulded  concrete,  such  as  is  used  for  building  blocks,  the 
mixture  is  kept  as  dry  as  possible  in  order  that  it  may  be  rapidly 
turned  out  of  the  moulds.  Even  in  such  work,  however,  it  is 


CONSISTENCY  OF  CONCRETE  173 

undesirable  to  have  too  little  water,  as  a  somewhat  larger 
proportion  produces  a  stronger  article  with  a  better  surface. 

The  best  consistency  of  a  concrete  mixture,  like  that  of  a 
clay  used  by  the  potter,  is  easily  recognised  by  men  accustomed 
to  working  the  material,  but  it  is  almost  impossible  to  describe 
it  with  accuracy  or  to  express  the  proportions  of  solid  material 
and  water  in  terms  of  definite  figures  (see  p.  171). 

Most  of  the  water  added  to  a  concrete  mixture  is  purely 
mechanical  in  its  action  ;  the  proportion  required  to  effect 
the  hydrolysis  of  the  cement  being  small.1  Nevertheless, 
this  additional  water  performs  several  very  important  duties, 
and  to  limit  its  amount  unduly  is  to  reduce  the  strength 
of  the  concrete.  Thus,  whilst  it  is  usually  necessary  to  add 
about  10  per  cent,  of  water  to  the  concrete  in  order  to  obtain 
a  workable  mass,  a  considerable  proportion  of  this  water  is 
removed  on  tamping.  Some  experiments  made  to  ascertain 
this  proportion  showed  that  it  is  usually  about  one-quarter  of 
the  water  originally  added,  but  with  very  wet  mixtures  it  may 
amount  to  half.  Most  of  the  water  remaining  immediately 
after  tamping  is  removed  by  evaporation  as  the  structure  dries, 
and  only  about  1  per  cent,  is  left  in  combination  in  the  dry 
concrete. 

MIXING. 

The  various  components  of  concrete  must  be  thoroughly  and 
rapidly  mixed  in  order  to  form  a  uniform  mass  before  the  cement 
has  begun  to  set.  To  prolong  the  mixing  is  seriously  detri- 
mental to  the  concrete,  just  as  cement  which  has  been 
"  worked "  too  much  in  making  tests  (p.  133)  is  greatly 
reduced  in  strength.  Two  main  methods  of  mixing  are 
employed,  viz.,  by  hand  and  by  means  of  mechanical  mixers. 
The  latter  have  the  advantage  of  ensuring  a  fairly  thorough 
mixture  and  avoid  any  possibility  of  "  scamping  "  the  labour, 
such  as  not  infrequently  occurs  in  hand  mixing.  When  skilled 
and  conscientious  men  are  employed,  however,  the  concrete 
they  produce  is  better  than  that  produced  by  any  machine, 
but  the  labour  of  mixing  large  quantities  by  hand  is  so  great 

1   It   has   been   shown   experimentally  that   the    water   required   for  com 
hydration  is  about  14  per  cent,  of  the  weight  of  the  cement  used  (see  p.  89). 


174     THE  PREPARATION  OF  CONCRETE 

and  it  is  so  difficult  to  rely  on  the  men  usually  employed  for 
this  purpose,  that  machine-mixing  is  now  employed  on  all 
large  work.  Whichever  method  is  used,  it  is  of  the  greatest 
importance  that  the  proportions  of  the  various  ingredients  are 
as  accurate  as  possible,  and  that  the  measures  used  are  correct 
in  size  and  are  properly  employed.  The  mixing  must  be  so 
thorough  that  the  product  is  uniform  in  colour  and  in  texture. 

Hand  mixing  is  the  most  effective  if  properly  carried  out, 
but  this  can  only  be  done  when  relatively  small  quantities  of 
concrete  are  required.  The  materials  should  be  turned  over 
three  or  four  times  and  well  mixed  in  the  dry  state,  these 
operations  being  repeated  to  an  equal  extent  after  the  addition 
of  the  water.  The  mixing  should  be  made  on  a  water-tight  and 
non-porous  platform  about  nine  feet  by  ten  feet,  constructed 
of  one-inch  boards  cleated  together  and  provided  with  a  frame 
to  keep  any  surplus  water  from  running  off  the  platform.  The 
surface  of  this  platform  should  be  planed  smooth.  This 
mixing  board  should  be  placed  as  near  as  possible  to  the  spot 
where  the  cement  is  to  be  used,  and  in  such  a  position  that  the 
men,  in  mixing,  move  their  shovels  along,  and  not  across,  the 
joints  between  the  boards.  The  board  must  be  packed  so 
that  it  is  solid  and  level  before  use.  Any  tendency  to  sag  in 
the  centre  must  be  overcome  by  packing  with  sand  or  ballast. 

A  suitable  quantity  of  sand  should  be  measured  by  means 
of  the  measuring  box  (p.  168).  The  sand  is  shovelled  in  without 
any  beating  or  packing  until  the  box  is  more  than  full.  The 
excess  of  sand  is  then  removed  by  laying  a  stout  lath  or  iron 
bar  on  top  of  the  box  and  drawing  it  along.  By  repeating  this 
action  two  or  three  times,  the  box  will  be  filled  just  level  with 
its  upper  edges.  The  measured  sand  is  then  turned  out  on  to 
the  measuring  board  and  spread  into  a  3-inch  or  4-inch 
layer.  On  top  of  the  sand  the  requisite  number  of  cement 
bags  is  emptied  as  evenly  as  possible.  Two  men,  standing  at 
opposite  sides  of  the  board,  then  mix  the  sand  and  cement 
together,  turning  the  material  over  and  over  so  as  to  mix  it 
thoroughly.  The  sand  and  cement  should  be  turned  over 
thrice,  a  shovelful  at  a  time,  after  which  it  should  be  sufficiently 
mixed  for  the  aggregate  and  water  to  be  added.  It  is  next 
spread  as  evenly  as  possible  on  the  board,  and  the  larger 


MIXING  CONCRETE  175 

measuring  box  (p.  168)  is  then  filled  with  aggregate  in  the 
same  manner  as  the  sand,  care  being  taken  that  the  box  is 
filled  "  just  level  "  and  not  "  heaped  up."  The  measured 
quantity  of  aggregate  is  then  turned  onto  the  sand  and  cement 
mixture  as  evenly  as  possible.  About  three-quarters  of  the 
water  likely  to  be  needed  is  next  thrown  on  to  the  aggregate, 
and  the  whole  mass  is  then  turned  over  and  over,  one  shovelful 
at  a  time,  water  being  added  to  the  drier  portions  until  the 
whole  of  the  water  has  been  added.  With  skilled  men  the 
mixing  will  be  complete  after  three  turnings,  but  if  it  shows 
streaks  or  dry  portions  it  must  be  turned  again.  It  is  then 
shovelled  into  a  compact  pile  and  is  ready  for  use.  It  must 
not  be  kept  long,  nor  must  much  time  have  been  occupied  in 
the  mixing,  or  the  cement  will  have  commenced  to  set  and  the 
concrete  will  be  spoiled. 

Re-mixed  concrete  must  never  be  used. 

Where  the  aggregate  consists  of  a  naturally  occurring  mixture 
of  sandy  gravel,  this  should  be  measured  out  and  spread  on 
the  board,  wetted  thoroughly,  and  then  covered  with  the 
proper  quantity  of  cement.  The  mixture  is  then  turned  over 
three  separate  times,  as  before,  the  additional  water  not  used 
for  the  sand  being  added  during  the  turning.  It  requires  a 
considerable  amount  of  skill  to  mix  cement  with  sandy  gravel, 
and  it  is  usually  better  to  screen  the  materials  and  mix  the 
sand  and  cement  and  add  the  gravel  and  water  later. 

It  is  never  satisfactory  to  mix  the  sand  and  aggregate  and 
to  add  the  cement  later  ;  the  sand  and  cement  should  first 
be  mixed,  as  this  secures  a  more  homogeneous  product  and 
yields  a  stronger  concrete. 

Too  much  care  cannot  be  taken  in  securing  the  adequate 
supervision  of  the  men  employed  in  mixing  concrete,  and  in 
all  cases  the  men  themselves  should  be  exceptionally  reliable 
and  trustworthy.  The  mixing  of  concrete  is  hard  and  laborious 
work,  and  is  severely  straining  to  the  wrists.  Moreover,  the 
greater  part  of  the  labour  can  be  avoided  by  moving  the 
material  without  turning  it  over.  This  is  a  dodge  to  which 
careless  and  unscrupulous  men  resort,  yet  it  is  fatal  to  the 
successful  mixing  of  the  materials,  and  is  one  of  the  strongest 
arguments  in  favour  of  the  use  of  mechanical  mixers. 


176     THE  PREPARATION  OF  CONCRETE 

Each  day,  at  the  conclusion  of  the  mixing,  the  board  should 
be  carefully  cleaned  and  freed  from  all  cement,  aggregate  and 
sand.  This  is  best  accomplished  by  first  sweeping  and  then 
scrubbing  it.  If  the  board  is  not  properly  cleaned  the 
shovelling  will  be  made  much  harder  on  later  days. 

Mechanical  mixers  may  be  divided  roughly  into  two  main 
divisions  :  (1)  continuous  mixers,  in  which  the  material  is 
continuously  fed  and  issued  from  the  machine  ;  and  (2)  batch 
mixers,  in  which  a  certain  quantity  of  concrete  is  fed  and  mixed 
and  then  discharged  as  a  batch.  There  are  advocates  for  both 
types  of  machine,  and  certainly,  if  operated  correctly,  both 
can  do  excellent  work. 

Batch  mixers  are  filled  with  measured  quantities  of  the 
materials,  are  then  set  in  operation  for  a  given  time  and  finally 
discharge  a  batch  of  concrete  ready  for  use. 

The  designing  of  a  concrete  mixer  which  will  give  the 
requisite  number  and  kind  of  movements  required  for  the 
making  of  good  concrete,  is  by  no  means  a  simple  matter,  as 
the  long  trail  of  failures  has  taught  the  manufacturer  and 
contractor  to  their  cost.  A  concrete  mixer  must  be  designed 
to  operate  under  the  severest  conditions,  and  the  mechanical 
construction  should  therefore  be  of  the  best,  while  the  great 
rapidity  of  output  required  for  modern  building  operations 
calls  for  a  machine  which  will  mix  and  discharge  a  batch  in 
the  shortest  possible  time. 

In  the  Smith  mixer  (the  T.  L.  Smith  Co.,  Ltd.)  and  the 
roll  mixer  (Builders  and  Contractors'  Plant,  Ltd.)  the  box 
consists  of  a  double  cone  which  can  be  revolved  on  its  own 
axis  to  discharge  the  material  at  one  side  ;  in  the  Ransome 
mixer  the  rotating  box  is  a  short  cylinder  with  blades  to  mix 
the  materials.  The  Express  mixer  (U.  K.  Winget  Concrete 
Machine  Co.)  consists  of  the  open  circular  pan  with  blades  to 
mix  the  materials,  the  discharge  being  effected  by  opening 
doors  in  the  bottom.  A  small  mixer  consisting  of  an  open  box 
or  trough  with  paddles  is  also  made  by  the  Ransome  Van  Mehr 
Machinery  Co.  The  objection  to  paddles  is  the  liability  of 
stones  to  jam  them,  but  this  may  be  avoided  by  the  provision 
of  safety  springs. 

It  is  important  that  the  discharge  from  machines  should  be 


MECHANICAL  MIXERS 


177 


rapid  or  the  tendency  of  the  materials  to  become  un-mixed  will 
reduce  the  strength  of  the  cement. 

In  the  construction  of  the  Panama  Canal  a  machine  known 
as  the  "  Chicago  improved  cube  mixer  "  is  used.  This  is  a 
simple  adaptation  of  the  first  form  of  machine  used  extensively 
in  the  concrete  industry — namely,  the  cubical  box,  journalled 
at  diagonally  opposite  corners,  and  having  a  door  on  one 
side  through  which  the  charge  of  cement,  sand,  stone,  and 
water  were  filled  ;  the  machine  was  rotated  for  several 


FIG.  32. — Chicago  Cube  Mixer,  used  in  Panama  Canal. 

minutes,  and  the  batch  of  mixed  concrete  then  emptied 
out.  Though  mechanically  crude,  these  machines  produced  an 
excellent  concrete,  the  main  objections  to  them  being  that, 
to  discharge,  the  cube  had  to  be  stopped  with  the  door  at  the 
bottom  and,  to  be  recharged,  had  to  be  turned  until  the  side 
containing  the  door  came  to  the  top — further  time,  of  course, 
being  lost  in  unclamping  and  reclamping  the  door. 

In    the    improved  forms   of   this   machine    (Figs.   32 — 37), 
which  preserves  the  principle  of  treating  the  batch  as  a  unit, 

C.  N 


178     THE  PREPARATION  OF  CONCRETE 

and  include  both  longitudinal  and  rotary  movements,  and  mix 
by  knead'ng  and  not  by  stirring,  many  interesting  modifica- 
tions have  been  introduced.  For  example,  the  shaft  of  the 
older  machines  is  replaced  by  hollow  trunnions  riding  on 
rollers,  and  made  sufficiently  large  to  serve  as  openings  for 
charging  and  discharging.  To  rotate  the  cube,  a  strong 
circumferential  rack  is  fastened  around  it  at  right  angles  to 
and  midway  between  the  trunnions  ;  and  this  rack,  geared  with 
a  pinion  shaft,  is  so  operated  by  the  engine  shaft  that  all 
gearing  is  removed  as  far  as  possible  from  the  material  which 


FJG.  33.— T.  L.  Smith  Co.,  Ltd.,  Power  Mixer. 

flies  about  during  the  charging  and  discharging  operations. 
An  automatic  dumping  device  has  been  provided,  and,  to 
ensure  the  construction  of  a  larger  cube  in  the  same  space,  and 
eliminate  any  opportunity  for  pocketing  the  fine  mortar,  the 
former  sharp  corners  and  edges  are  rounded. 

The  cube  in  Fig.  32  is  provided  with  breaker  rods,  from 
|  inch  to  1  inch  in  diameter,  so  placed  across  each  of  the  six 
central  corners  of  the  cube  that  as  the  machine  revolves  they 
slice  through  the  mass  of  concrete,  each  following  a  different 


MECHANICAL  MIXERS 


179 


line,  and  effectually  breaking  any  hard  lumps  or  cakes  found- 
in  the  materials.  Otherwise,  the  interior  of  the  cube  is  free 
from  paddles,  deflectors,  or  other  obstructions. 

In  the  mixer  made  by  the  T.  L.  Smith  Co.,  Ltd.  (Figs.  33 
and  34),  the  drum  consists  of  two  cones  and  contains  blades 
arranged  spirally.  In  the  Victoria  mixer  supplied  by  the  same 
firm  (Figs.  35  and  36)  the  drum  is  cylindrical,  with  four 
deflecting  blades  which  subject  the  concrete  to  twelve  distinct 
mixing  actions  for  each  revolution  of  the  machine.  A  dis- 
charging spout  (Fig.  36)  can  be  swung  partly  into  the  cylinder 
when  required.  The  use  of  the  skip  (Fig.  35)  enables  the 
mixer  to  be  loaded 
from  the  ground  level. 
Above  the  machine  is 
a  tank  (Fig.  35)  for 
the  automatic  supply 
of  water  to  the 
concrete. 

The  Ransome-ver 
Mehr  Machinery  Co., 
Ltd.,  make  a  large 
number  of  different 
types  of  concrete 
mixers,  and  their 
machines  are  exten- 
sively used  in  all 
parts  of  the  United 
Kingdom.  Fig.  37 
shows  one  of  them 
with  an  engine  and  boiler  directly  coupled  to  the  mixer. 

Where  batches  of  only  two  cubic  feet  each  are  required,  or 
where  a  power-driven  mixer  is  not  available,  a  hand  mixer 
may  be  used. 

In  the  type  of  hand  mixer  (Fig.  38)  supplied  by  the  Ransome- 
ver  Mehr  Machinery  Co.,  Ltd.,  the  aggregate  is  fed  into  an 
elevating  skip  by  the  operator,  the  skip  itself  having  a  capacity  of 
two  cubic  feet  of  material.  The  mouth  of  the  skip  is  extended 
in  the  shape  of  a  chute,  in  order  to  ensure  no  spilling  of  material 
when  the  skip  discharges  into  the  mixing  drum,  the  necessary 


FIG.  34. — Cut-away  View  showing  interior 
of  Fig.  33. 


180 


THE  PREPARATION  OF  CONCRETE 


FIG.  35.— Victoria  Mixer,  with  Skip 
for  filling. 


water  being   added   simultaneously.      Materials   entering   the 

drum  immediately  get  into  contact  with  the  mixing  blades, 

which  are  of  a  special 
form,  designed  to 
ensure  the  whole  of  the 
batch  being  thoroughly 
mixed. 

The  mixing  opera- 
tion being  complete 
(this  usually  occupies 
about  30  seconds),  the 
drum  itself  is  inverted 
by  means  of  an  aux- 
iliary handle,  pinion 
wheel,  and  rack.  As 
the  drum  is  inverted  it 
discharges  its  contents, 
being  assisted  during 
the  whole  period  by 
the  action  of  the 

paddles  themselves.     The  paddles,  in  addition  to  facilitating 

discharge,    at  the  same  time  automatically  clean  the  drum. 

The  mixer  is  of  such 

dimensions      that      a    ":.   ••"•',  '-"" :    -  ~~  •••••• 

standard       navvy 

barrow  can  be  readily  W^ 

placed    beneath     the    : 

drum,     in     order     to 

receive      the      batch 

when    discharged. 

This     eliminates     all 

necessity     for    lifting 

material   into  the 

barrows    after    it     is 

mixed,  as  is  the  case 

with     ordinary    hand 

mixing. 

A  batch  mixer  of  a  different  type  is  known  as  the  "  Express  " 

mixer,  and  is  sold  by  the  U.  K.  Winget  Concrete  Machine  Co., 


FIG.  36.— Victoria  Mixer  (in  use). 


MECHANICAL  MIXERS  181 

Ltd.,  of  Newcastle-on-Tyne.  In  designing  this  machine,  the 
idea  kept  in  view  was  that  of  mixing  the  material  by  mechanical 
means  in  a  manner  precisely  similar  to  hand-mixing  properly 
conducted.  The  machine  (Figs.  39,  40)  will  mix  concrete  in  which 
the  aggregate  is  not  larger  than  one  and  a  half  inch  gauge  ;  it 
is  therefore  particularly  suitable  for  concrete  block  and  rein- 
forced work.  The  maximum  charge  is  five  cubic  feet  ;  t  his 
quantity  is  mixed  dry,  then  water  is  added  and  mixed  with 


KKJ.  37.— Banabme-ver  Mdir  Power  Mixer. 

the  concrete,  the  finished  product  being  delivered  from  the 
machine  on  to  the  ground  in  sixty  seconds.  The  "  Kx  press  " 
mixer  will  mix  either  dry,  semi -dry,  or  wet,  and  as  it  is  a  batch 
mixer  the  proportions  of  the  different  materials  may  be  altered 
at  will  to  suit  requirements.  The  power  required  to  drive  is 
about  6  b.h.p.,  so  that  the  "  Express  "  mixer  is  eheap  to  drive  ; 
it  has  nothing  to  get  out  of  order,  and  if  the  ploughs  get  worn 
out  they  can  be  replaced  at  a  small  expense.  Each  part  of 
the  machine  is  accessible  for  cleaning,  and  it  does  not  clog  up. 


182 


THE  PREPARATION  OF  CONCRETE 


Fig.  39  gives  a  general  view  of  the  machine,  which  consists 
of  a  stationary  pan  6  feet  6  inches  diameter  and  1  foot  9  inches 
deep.  The  capstan  is  keyed  on  to  a  vertical  shaft,  driven  from 
below  by  mitre  gearing,  and  carries  revolving  arms,  to  which 
are  attached  adjustable  plough-shaped  beaters  ;  each  plough 
carries  in  its  rear  an  iron  rake.  The  action  of  the  machine  is 
to  imitate  hand-mixing,  but  the  number  of  times  the  materials 


FIG.  38. — Hand  Mixer  for  Concrete. 

are  turned  over  in  the  pan  is  enormously  greater  than  in  the 
most  patient  hand- mixing.  This  is  caused  by  the  combined 
action  of  the  ploughs  turning  over  the  material,  and  the  rake 
immediately  following  raking  it  out  ready  for  the  next  succeed- 
ing plough  to  turn  over.  In  some  mixers  of  this  type  the 
materials  are  piled  up  in  great  furrows  where  the  large  material 
falls  from  the  top  of  the  furrow  to  the  bottom,  exactly  as 
happens  in  badly  conducted  hand  mixing,  but  in  the  "  Express" 


MECHANICAL  MIXERS 


183 


FIG.  39.— Top  view  ol 
Express  Mixer. 


mixer  the  material  is  kept  level  in  the  pan.     This  machine 

mixes    without    balling    the    material — a    most    objectionable 

thing  in  concrete  block  work. 

If  a  continuous  mixer  is  used  it  must  be  so  constructed  that 

it  cannot  be  tampered  with.     Some  patented  mixers  of  the 

continuous  type  do  not   mix  the 

material      sufficiently,      and      so 

produce    a    poor    and     irregular 

concrete.     For  this   reason  batch 

mixers  are  often  preferred.     With 

the  latter,  the  same  precaution  is 

necessary,  as  some  of  them  permit 

the  material  to  be  fed  in  at  the 

same   time   as    other    material    is 

being    discharged,   thereby   intro- 
ducing   some    imperfectly    mixed 

material     into     the     waggon     or 

wheelbarrow    intended     to     receive     the     mixed     concrete. 

In  Barker  and  Hunter's  mixer,  the  simultaneous  filling  and 

discharging  of  the  machine  is  made  impossible  by  the  necessity 

of  rotating  the   machine  in   one   direction   when   filling  and 

mixing,  and  in  the  opposite  one  when  discharging. 

A  discussion  of  the 
relative  merits  of  the 
various  types  of  mixers 
is  beyond  the  scope  of 
the  present  volume,  but 
care  should  be  taken  in 
selecting  a  machine  to 
choose  one  in  which  there 
is  no  likelihood  of  the 
particles  being  separated 

FIG.  40.— Side  view  of  Express  Mixer,      instead   of   being    mixed 

more  completely.     There 

is  some  danger  of  this  in  machines  fitted  with  internal  blades. 

It  is  also  necessary  to  keep  the  machine  in  good  working  order. 
As  it  is  essential  that  the  output  of  the  mixer  should  approxi- 
mate that  at  which  it  is  rated  by  the  makers,  these  machines 
should  always  be  purchased  under  guarantee  from  a  reliable 


184     THE  PREPARATION  OF  CONCRETE 

firm.  The  effect  of  the  work  of  mixing  is  very  severe  on  the 
machines,  and  it  is  often  aggravated  by  neglect.  The  best 
way  is  to  put  a  competent  man  in  charge  of  the  mixer  and 
to  make  him  responsible  for  it.  He  should  be  allowed  at 
least  half  an  hour  a  day  for  cleaning  the  machines,  tightening 
nuts  and  keys,  and  attending  to  the  bearings,  etc. 

Whatever  method  or  machine  is  used  it  is  wise  always  to 
mix  the  cement  and  aggregate  well  before  adding  the  water, 
then  to  mix  thoroughly  after  the  water  has  been  added,  great 
care  being  taken  not  to  work  through  the  initial  set. 

It  is  desirable  not  to  mix  concrete  during  frost,  but  if  this 
is  unavoidable  the  use  of  warm  water  for  gauging  and  of  warm 
cement  and  aggregate  will  reduce  the  risks  of  working  at 
undesirably  low  temperatures.  Some  suggestions  with  regard 
to  this  will  be  found  later  under  the  caption,  "  Placing  the 
Concrete." 

MOULDS  AND  CENTERING. 

Freshly  mixed  concrete  is  a  plastic  material  which  can  be 
moulded  into  any  shapes  consistent  with  the  size  of  its  particles. 
To  obtain  any  given  shape,  however,  some  kind  of  mould  or 
form l  is  necessary,  and  the  concrete  paste  is  poured  into  this 
and  allowed  to  set.  The  hard  mass  is  then  removed  from  the 
mould,  or,  more  commonly,  the  mould  is  taken  away  piece- 
meal, leaving  the  moulded  mass  standing  and  increasing  in 
strength  and  hardness  with  the  lapse  of  time. 

For  most  structural  purposes,  the  forms  used  in  concrete 
construction  are  built  of  timber,  the  boards  being  used 
separately  or  in  groups  (termed  shutters)  where  a  number  of 
forms  of  the  same  size  are  required  for  consecutive  use.  Great 
ingenuity  is  sometimes  exercised  in  providing  forms  for 
structures  of  special  shapes,  but  for  most  purposes  temporary 
walls  of  wood  are  built  on  the  spot  where  the  concrete  wall  or 
other  structure  is  desired.  These  wooden  "  walls  "  or  shutters 

1  It  is  interesting  to  note  that  the  word  form  was  introduced  into  this  country 
from  the  United  States.  It  owes  its  origin  to  the  German  emigrants,  the  word 
Form  being  the  German  equivalent  of  the  English  word  mould.  In  modern  concrete 
construction  the  use  of  the  word  mould  is  limited  to  the  moulds  used  for  com- 
paratively small  articles,  and  indicates  a  permanent  piece  of  apparatus.  Where  a 
"  mould  "  is  built  up  and  taken  down  after  use  it  is  termed  a,  form.  Sometimes  the 
terms  shuttering  or  centering  are  used  instead  of  mould  and  form. 


FORMS 


185 


are  placed  the  same  distance  apart  as  the  desired  thickness  of 
the  concrete,  and  they  are  supported  by  props  and  angle  pieces 
so  as  to  remain  secure  against  the  thrust  of  the  concrete. 

The  construction  of  the  form  for  a  square  column  and  floor 
is  shown  in  Fig.  41. 

Three  important  precautions  must  be  taken  :  the  forms  must 
be  removable  without  jarring  the  concrete,  there  must  be  no 
leakage  between  the  boards  constituting  the  form,  and  the 
whole  structure  must  be  stiff  enough  not  to  bulge  or  alter  in 
shape  during  use.  To  aid  this,  perforated  wooden  plugs  or 
separators,  with  bolts  through  them,  may  be  used,  the  bolts 
and  wood  being  removable,  and  the  holes  in  the  concrete  filled 
up.  Much  concrete  work  which  would  otherwise  be  excellent 
is  spoiled  by  insecure  and  insufficiently  rigid  forms  and 


FIG.  41. — Arrangement  of  Forms. 

centering,  and  it  is  therefore  of  great  importance  that  special 
attention  should  be  paid  to  this  part  of  the  work. 

Boards  one  inch  in  thickness  are  usually  strong  enough,  but 
they  must  be  supported  by  a  sufficient  number  of  shores  or 
buttresses,  the  latter  being  also  of  wood  and  placed  about  two 
feet  apart.  Thick  boards  last  longer  and  are  cheaper  in  the  end. 

The  cost  of  forms  is  very  great,  and  unless  care  is  taken  in 
constructing  them  much  waste  may  result.  By  careful 
planning  beforehand,  most  of  the  boards  may  be  used 
repeatedly,  and  for  small  work  they  need  cost  practically 
nothing,  being  made  of  odds  and  ends  of  timber,  which  is  as 
valuable  afterwards  as  before  it  was  used. 

The  construction  of  the  form  requires  great  care,  as  mistakes 
cannot  be  remedied.  The  space  inside  should  also  be  kept 


186     THE  PREPARATION  OF  CONCRETE 

clean,  any  chips,  etc.  being  removed  at  once.  Where  possible 
the  shape  should  be  such  that  the  angles  of  the  concrete  are 
rounded. 

The  essential  characteristics  of  good  forms  are  rapidity  and 
ease  of  construction,  great  stability  in  use  and  rapidity  and 
ease  of  removal  without  tearing  or  jarring  the  concrete. 

The  wood  should  be  wetted  before  pouring  in  the  concrete, 
so  as  to  prevent  the  adhesion  of  the  latter.  Unless  this  is 
done,  some  of  the  concrete  will  be  torn  away  when  the  forms 
are  taken  down.  For  special  work  the  forms  may  be  lime- 
washed,  oiled  or  coated  with  soft  soap.  Creosote  and  kerosene 
oil  are  useless,  but  linseed  and  black  or  cylinder  oils  are 
excellent.  Even  when  oil  is  used,  it  is  desirable,  just  before 
pouring  in  the  concrete  to  flush  the  form  with  water. 

The  designing  and  construction  of  forms  is  in  itself  a  large 
subject,  and  to  enter  upon  it  in  further  detail  is  beyond  the 
scope  of  the  present  volume.  It  should  be  observed,  however, 
that  the  use  of  forms  would  be  greatly  facilitated  and  the  cost 
greatly  reduced  if  greater  standardisation  were  possible,  though 
the  awkwardly-shaped  and  restricted  sites  upon  which  it  is 
so  often  necessary  to  work  appear  to  prohibit  this  in  the 
majority  of  cases.  Even  under  good  conditions  the  centering 
costs  one-third  of  the  total  price  for  the  concrete,  and  may 
easily  be  more  than  this,  so  that  any  saving  which  may  be 
effected  is  of  considerable  influence.  At  present  the  chief 
economy  is  realised  by  avoiding  all  unnecessary  cutting  of 
the  boards  and  by  planning  out  the  work  in  such  a  manner 
that  the  long  lengths  of  timber  can  be  used  repeatedly.  By 
making  beams,  floor  panels  and  columns  of  certain  standard 
sizes,  and  adhering  to  these  as  far  as  ever  possible,  the  cost  of 
forms  may  be  reduced  to  5  per  cent,  for  firms  in  regular  work. 
The  timber  is  not  damaged  by  the  concrete,  but  in  most 
instances  it  is  cut  up  into  short,  useless  pieces  which  cannot 
be  used  again,  and  have  to  be  replaced  by  new  ones.  Architects 
and  engineers  may  often  effect  a  great  saving  by  using  pillars 
of  standard  size  and  placing  them  standard  distances  apart. 
The  question  of  the  use  of  standard  forms  is  one  to  which  the 
student  of  concrete  construction  may  well  devote  a  large 
amount  of  thought  and  care. 


PLACING  CONCRETE  187 

The  operation  which,  in  the  metal  industries,  is  known  as 
casting,  and  in  the  plastic  industries,  as  moulding,  is,  in  concrete 
construction,  termed  placing.  In  each  case  it  is  the  operation 
which  gives  the  material  the  shape  which  it  is  desired  it  shall 
possess. 

PLACING     CONCRETE. 

Immediately  after  the  concrete  has  been  properly  mixed  it 
should  be  placed  in  the  forms  or  moulds  or  on  the  spot  in  which 
it  is  to  be  used.  There  should  be  no  delay  in  placing  the 
concrete  or  the  consequences  may  be  serious.  It  is,  therefore, 
wise  to  mix  the  material  in  only  small  amounts,  and  on  no 
account  to  use  concrete  which  has  begun  to  set.  A  slow-setting 
cement  should  be  used  when  the  concrete  has  to  be  taken  a 
considerable  distance  from  the  place  of  mixing  to  where  it 
can  be  poured  into  the  forms.  The  concrete  paste  may  be 
handled  and  "  placed  "  in  any  convenient  manner,  providing 
that  it  does  not  begin  to  set  before  it  is  in  position  and  that 
no  un-mixing  of  the  material  occurs.  Wherever  possible,  the 
concrete  should  be  shovelled  direct  from  the  mixing  board  or 
machine  into  the  forms,  but  where  large  quantities  are  required 
it  is  usually  necessary  to  transport  it  in  barrows  to  that  part 
of  the  structure  where  it  is  required. 

A  wheelbarrow  holding  two  cubic  feet  of  concrete  is  an 
exceedingly  heavy  load,  and  where  the  concrete  is  very  wet, 
a  load  of  one  cubic  foot  is  not  uncommon,  since  the  ordinary 
steel  or  wooden  barrow  has  a  body  or  bowl  too  shallow  to 
prevent  the  wet  concrete  from  overflowing.  To  reduce  the  cost 
of  transporting  concrete,  the  Ransome-ver  Mehr  Machinery  Co., 
Ltd.,  have  designed  an  all-steel  cart  (Fig.  42)  that  holds  six  cubic 
feet  (water  measure).  One  man  can  push  or  pull  this  cart 
over  a  plank  runway,  even  when  the  cart  is  level  full  of  concrete. 
In  other  words,  one  man  transports  from  three  to  six  times  as 
much  concrete  as  he  could  transport  in  a  wheelbarrow.  This 
remarkable  result  is  due  to  the  wheels  of  the  cart  being  much 
larger  than  those  of  a  wheelbarrow,  and,  therefore,  more  easy 
running  is  secured  ;  no  weight  is  thrown  on  to  the  man's 
hands  as  in  the  case  of  a  wheelbarrow,  but  he  is  free  to  use  all 
his  strength  in  pushing  or  pulling  the  cart,  and  as  no  concrete 


188 


THE  PREPARATION  OF  CONCRETE 


is  slopped  on  to  the  run-planks  where  these  carts  are  used, 
only  half  the  effort  is  needed  to  push  a  cart  over  clean  planks 
that  is  necessary  when  going  over  dirty  ones.  In  addition  to 
the  larger  loads  moved  per  man,  there  is  an  important  economic 
advantage  in  being  able  to  discharge  the  batch  from  a  concrete 
mixer  in  much  less  time  where  these  carts  are  used  instead  of 
wheelbarrows,  as  a  mixer  can  be  discharged  into  these  carts 
in  one-third  the  time  required  with  wheelbarrows. 


FIG.  42.— Cart  for  Concrete. 

The  amount  of  concrete  added  at  a  time  should  not  be  more 
than  will  produce  a  layer  about  six  inches  in  thickness,  or 
three  inches  in  the  neighbourhood  of  reinforcement.  Other 
things  being  equal,  the  strength  of  concrete  depends  on  its 
compactness  or  density.  Hence  it  is  desirable  to  use  some 
means  of  increasing  this.  A  vertical  spade  is  then  inserted  into 
this  layer  close  to  the  inner  face  of  the  form,  and  is  worked 
up  and  down  so  as  to  push  the  aggregate  away  from  the  form, 
release  any  air  bubbles  and  produce  an  even  face.  The  size 


PLACING  CONCRETE 


189 


of  the  spade  will  depend  on  the  space  into  which  it  is  introduced ; 
a  long  board  four  inches  by  one  inch,  sharpened  to  a  chisel 
edge,  is  exceedingly  useful  for  this  purpose,  though  for  broader 
work  an  ordinary  spade  may  be  used.  Spading  in  this  manner 
needs  considerable  skill  if  a  dry  concrete  mixture  is  used  ;  with 
a  very  sloppy  mixture,  on  the  contrary,  no  such  treatment  is 
needed,  though  it  is,  in  all  cases,  a  wise  precaution.  A  ram 
or  tamping  tool  (Fig.  43)  is  then  held  just  above  the  top  of 
the  concrete  layer  and  is  brought 
down  into  it  in  a  succession  of 
blows  until  all  the  material  is  com- 
pact and  the  surplus  water  has 
risen  to  the  surface.  Hard  ramming 
is  seldom  necessary  with  a  well- 
made  concrete,  though  the  drier 
the  mixture  the  harder  must  be 
the  blows.  Indeed,  very  heavy 
blows  tend  to  do  more  harm  than 
good,  what  is  required  being  a 
series  of  tamps  or  taps  of  just 
sufficient  force  to  secure  the  various 
particles  all  fitting  into  their  places 
without  leaving  any  voids.  Many 
light  taps  are  far  more  useful  than 
a  few  heavy  ones  in  consolidating 
the  concrete.  Some  firms  prefer 
to  use  a  wet  mixture  and  not  to 
tamp  at  all,  but  to  rely  on  the 
natural  fluidity  of  the  concrete 
aided  by  the  use  of  a  special 
spade  (Fig.  44).  Vibrators  - 

operated  in  a  similar  manner  to  pneumatic  hammers — are 
applied  to  each  side  of  the  form  or  shuttering  by  the 
Vibrocel  Co.,  Ltd.  This  special  mode  of  tamping  is  claimed 
by  the  patentees  to  give  a  more  impervious  concrete  than  is 
obtainable  by  any  other  method.  The  resumption  of  placing 
(after  an  interval)  and  the  attachment  of  new  concrete  to  old 
is  also  greatly  facilitated  by  vibrating  instead  of  the  more 
usual  tamping. 


FIG.  43.— Tamping  Tool. 


190     THE  PREPARATION  OF  CONCRETE 

If  the  work  of  placing  the  concrete  is  suspended,  all  necessary 
grooves  for  joining  future  work  must  be  made  before  the 
concrete  is  set.  When  the  placing  is  resumed,  the  previous 
concrete  must  be  wetted,  roughened,  cleaned  of  all  foreign 
material  and  covered  with  mortar,  consisting  of 
one  part  of  Portland  cement  with  not  more 
than  two  parts  of  sand  so  as  to  form  a  layer  of 
mortar  half  an  inch  thick.  Care  should  be 
taken  that  joints  of  this  kind  are  made  in 
positions  where  they  will  have  the  least 
possible  effect  on  the  strength  of  the  structure. 
Thus,  footings  and  floors  should  be  placed  the 
full  thickness  at  one  operation  ;  columns 
should  only  be  stopped  (if  at  all)  at  .  the 
underside  of  the  lowest  projection  of  the 
capital  ;  constructional  joints  in  beams  and 
girders  should  be  vertical  and  within  the  middle 
third  of  the  span,  and  similar  joints  in  slabs 
should  be  near  the  centre  of  the  span.  If 
"  plums  "  or  large  masses  of  stone,  or  packing 
is  used,  the  pieces  should  be  placed  in  position 
before  the  concrete  is  added.  These  large 
masses  should  be  at  least  two  inches  apart, 
and  should  not  be  within  two  inches  of  the 
face  or  back  of  the  form.  The  concrete  should 
be  rubbed  around  them  with  a  spade,  and 
that  immediately  above  and  around  them 
should  be  well  tamped. 

As  the  strength  of    the    structure  depends 
largely  on  the  care  and  skill   exercised   in  the 
placing  of   the  concrete,  this  should  have  all 
FIG.  44. — Spade  the   attention   it   needs.      Frost    has    a    very 
(Ross  )>nC1         serious  effect  on  plastic  concrete,  and  no  placing 
should  be  done  in  frosty  weather.     Under  some 
circumstances,  however,  working  at  a  temperature  below  32°  F. 
is  unavoidable,  and  the  concrete  must  therefore  be  protected 
against   the  action  of   frost    until   it   has  set  and  hardened. 
Where  the  concrete  will  not  be  seen  the  addition  of  common 
salt  or  calcium  chloride  to  the  water  used  for  mixing   will 


PLACING  CONCRETE  191 

reduce  the  freezing  point  of  the  water,  and  so  permit  the 
placing  to  go  on  as  usual.  Where  a  scummed  surface 
on  the  concrete  must  be  avoided,  it  is  necessary  to  heat  the 
materials  of  which  the  concrete  is  made  and  to  shelter  the 
concrete  until  it  has  set  and  become  fairly  hard.  The  tempera- 
ture of  the  materials  should  not  be  higher  than  that  of  boiling 
water,  and  the  water  itself  should  generally  be  only  lukewarm, 
and  certainly  not  above  100°  F.  Immediately  the  warm 
materials  have  been  mixed  and  the  concrete  placed  in  position, 
it  must  be  protected  by  canvas  tents  and  fires  or  hot  pipes. 

In  Canada,  good  results  are  obtained  by  spreading  the  whole 
of  the  materials  on  a  steam-heated  floor  and  by  using  hot 
water  for  mixing.  The  mixing  drum  is  also  heated  by  a 
steam  jacket. 

The  forms  are  made  of  sheet  iron  and  are  double,  steam  being 
led  between  the  walls  so  as  to  heat  the  core  or  space  to  be 
occupied  later  by  the  concrete.  During  this  preliminary 
warming  the  forms  are  covered  with  tarpaulin,  which  is 
removed  a  little  at  a  time  during  the  placing  of  the  concrete. 

Concrete  made  of  warm  materials  during  frost  is  never 
considered  to  be  quite  as  satisfactory  as  that  made  at  the 
normal  temperature.  Hence,  the  wisest  course  is  to  follow 
the  recommendations  of  the  joint  committee  under  the  auspices 
of  the  Royal  Institute  of  British  Architects,  viz.,  to  suspend 
all  work  during  frosty  weather,  to  protect  new  work  at  night 
when  frost  is  expected,  to  leave  the  centering  in  position  for 
a  fortnight  longer  than  ordinary,  and  not  to  remove  it  until 
all  signs  of  frost  have  departed. 

Placing  in  water. — Concrete  should  not,  as  a  rule,  be  allowed 
to  set  under  water.  Where  this  is  unavoidable,  special  pre- 
cautions must  be  adopted,  one  of  the  most  important  being 
to  prevent  the  cement  from  floating  away.  The  use  of  a 
drop-bottom  bucket  facilitates  rather  than  prevents  this  loss 
of  concrete.  In  no  case  should  soft  concrete  be  allowed  to 
drop  through  water. 

One  of  the  most  usual  methods  is  to  fill  the  concrete  into 
bags,  sewing  up  the  mouths  of  these  and  depositing  the  whole 
under  water.  This  method  involves  the  services  of  divers  and 
is  cumbersome,  slow  and  expensive  ;  it  is  rapidly  becoming 


192     THE  PREPARATION  OF  CONCRETE 

obsolete  except  for  repair  work  and  for  work  under  moving 
water. 

The  use  of  a  tremie  or  pipe  with  its  upper  end  projecting  out 
of  the  water  appears  to  have  many  advantages.  The  concrete 
is  poured  in  at  the  top  of  the  tube  as  fast  as  it  escapes  from  the 
lower  end.  In  practice,  this  method  presents  several  serious 
difficulties,  as  it  is  by  no  means  easy  to  prevent  the  water  from 
rising  inside  the  tube  and  floating  the  cement  away  from  the 
concrete.  The  motion  of  the  water  near  the  bottom  of  the 
tube  has  a  similar  action.  What  is  required  is  to  have  the 
lower  end  of  the  tube  buried  in  concrete  for  two  to  five  feet, 
so  as  to  form  an  effective  seal  against  the  outside  water  ;  when 
this  can  be  secured  the  use  of  tremies  is  highly  advantageous. 
The  tremie  is  raised  a  few  inches  at  a  time  as  the  work 
progresses. 

An  ingenious  device,  used  by  the  Vibrocel  Co.,  Ltd.,  consists 
of  polygonal  cells  of  concrete  which  are  made  on  land  and  then 
floated  to  the  place  where  they  are  to  be  used.  These  floating 
cells  are  then  kept  vertical  in  the  water  and  the  bottom  is 
blown  out  by  means  of  a  gelignite  charge  previously  placed 
therein.  The  cells  then  sink  to  the  bottom  of  the  water  and 
form  a  permanent  tremie  which  is  later  filled  with  concrete. 

The  concrete  should  be  wetter  than  that  employed  on  dry 
land,  in  order  that  it  may  flow  properly.  Care  should  be  taken 
not  to  disturb  the  freshly-set  concrete,  and  the  deposition  of 
the  concrete  should  be  continuous  so  as  to  avoid  the  necessity 
for  cleaning  the  surface  of  the  different  lots. 

Where  coffer-dams  are  used  they  should  be  sufficiently 
water-tight  to  prevent  a  current  of  water  through  the  pit,  and 
to  keep  any  water  in  the  pit  quite  still. 

SETTING  AND  HARDENING. 

The  chemical  and  physical  changes  which  take  place  during 
the  setting  and  hardening  of  concrete  are  practically  the  same 
as  those  occurring  in  cement.  If  the  concrete  has  been 
correctly  proportioned  and  prepared,  however,  there  will  be 
far  fewer  unhydrolysed  particles  of  cement  in  the  concrete 
than  when  neat  cement  is  used.  In  other  words  the  cemen- 


SETTING  AND  HARDENING  OF  CONCRETE      193 

titious  power  of  the  cement  is  more  fully  utilised  than  when 
no  sand  or  aggregate  is  present.  Apart  from  this,  as  the 
reactions  which  occur  take  place  exclusively  between  the 
cement  and  water,  there  is  no  need  to  describe  them  further  ; 
the  reader  who  desires  to  refresh  his  memory  concerning  them 
should  see  pp.  81  et  seq. 

If  the  "  sand  "  used  in  the  concrete  is  made  by  crushing 
bricks  and  burnt  clay  ballast  to  powder,  it  will  be,  of  itself, 
cementitious,  and  will  unite  with  the  free  lime  formed  by 
hydrolysis  of  the  Portland  cement,  and  will  form  a  pozzolanic 
cement  whieh  will,  in  its  turn,  be  hydrolysed  in  a  similar 
manner,  and  will  set  and  finally  harden  into  a  stony  mass. 
Properly  burned  clay  when  reduced  to  powder,  therefore, 
increases  the  strength  of  all  concrete  in  which  it  takes  the 
place  of  silicious  sand  (see  p.  159). 

Concrete  which  is  placed  under  water  sometimes  takes 
much  longer  to  set.  This  is  due  to  the  wetness  of  the  mixture 
rather  than  to  the  influence  of  the  water  in  which  it  is  immersed. 

STRIKING  CENTERING. 

The  removal  of  the  forms,  leaving  the  concrete  mass  in  situ, 
is  known  technically  as  "  striking  the  centering."  This 
operation  usually  takes  place  as  soon  as  the  concrete  has 
hardened  sufficiently  for  the  support  of  the  forms  to  be  no 
longer  necessary,  that  is  about  eight  to  ten'  days  after 
"  placing."  The  length  of  time  which  elapses  must,  however, 
be  left  to  the  discretion  of  the  man  in  charge  of  the  work,  and 
in  frosty  weather  the  forms  may  remain  for  three  weeks  or 
more.  The  longer  the  forms  remain  in  place  the  safer  will 
be  the  concrete.  Some  of  the  simpler  forms  may,  in  summer, 
be  removed  as  early  as  three  days  after  placing,  but  this  is 
somewhat  risky. 

The  side  casing  of  beams,  the  casing  of  columns,  and  for 
the  soffits  of  floor-slabs  of  less  than  five  feet  span,  may  usually 
be  removed  after  eight  days  ;  the  casing  of  soffits  of  beams 
and  floors  of  greater  span  should  not  be  removed  for  at  least 
fourteen  days,  whilst  for  arches  of  large  span  the  forms  should 
not  be  touched  for  at  least  a  month. 

c.  o 


194     THE  PREPARATION  OF  CONCRETE 

The  striking  of  centering  is  a  most  important  and  responsible 
duty,  as  if  done  too  soon  or  unskilfully  it  may  result  in  the 
collapse  of  the  structure  and  in  serious  loss  of  life.  Many  of 
the  fatal  accidents  in  connection  with  concrete — particularly 
in  America — have  been  caused  by  too  early  removal  of  the 
centering  and  by  loading  the  structure  before  the  concrete 
was  properly  hardened.  Concrete  is  often  used  for  bridges, 
floors,  and  in  other  positions  where  the  weight  of  material 
with  no  support  immediately  beneath  it  is  very  great,  so  that 
the  dead  weight  of  the  structure  alone  may  be  more  than 
half  that  of  the  live  load  it  is  designed  to  carry.  Consequently, 
the  premature  removal  of  the  forms  is  always  risky,  and  too 
much  care  and  skill  cannot  be  exercised  in  preventing  accidents 
due  to  this  cause. 

The  greatest  care  should  also  be  taken  in  removing  the  forms 
not  to  jar,  shake  or  tear  the  concrete,  as  it  will  not  be  fully 
hardened  and  may  easily  be  damaged.  After  the  removal  of 
the  forms,  the  concrete  should  be  protected  from  the  sun's 
rays,  rain,  dust  and  wind  by  canvas,  burlap  or  sheeting,  and 
its  surface  should  be  kept  wet  by  sprinkling  water  on  it  twice 
daily  for  five  or  six  days.  This  treatment  is  necessary  to 
prevent  the  outside  of  the  mass  drying  more  rapidly  than  the 
inside,  and  so  causing  strains.  In  summer  weather  and  tropical 
climates  this  watering  of  the  surfaces  should  be  done  with 
care  and  intelligence.  If  sheeting  or  burlap  is  used,  this 
should  be  wetted  as  well  as  the  surface  of  the  concrete. 

The  forms,  after  removal  from  the  structure,  should  be  at 
once  cleaned  by  means  of  a  short-handled  hoe,  care  being  taken 
not  to  gouge  the  wood. 

SURFACE  TREATMENT. 

Concrete  is  subject  to  many  conditions  which  make  it 
difficult  to  obtain  a  satisfactory  finish.  Every  irregularity  and 
almost  every  joint  in  the  forms  leaves  an  imprint.  Patches 
of  exposed  aggregate  show  here  and  there,  and  variations  of 
colour  occur  in  streaks  and  layers.  Thus,  a  discoloration  of 
the  material  may  be  due  to  (a)  partial  bleaching  of  the  lime 
compounds,  (b)  the  formation  of  efflorescence  or  "  scum," 


SURFACE  TREATMENT  OF  CONCRETE          195 

(c)  the  inclusion  of  organic  matter  in  the  water  used,  and  (d)  a 
"  laitence  "  face,  caused  by  an  excess  of  water  in  some  portion 
of  the  concrete  mixture  which  has  floated  some  of  the  lighter 
cement  particles  to  the  surface  and  produced  a  thin  im- 
permeable coating.  Roughness  or  irregularity  of  the  surface 
may  be  due  to  careless  work,  too  long  a  time  between  watering 
the  form  and  placing  the  concrete,  insufficient  "  spading  "  or 
rough  removal  of  the  forms  and  shuttering,  the  accidental 
inclusion  of  sawdust,  chippings,  etc. 

Untouched  concrete  work  may  normally  have  one  of  three 
distinctive  surfaces  : — 

(1)  Dense  surfaces  produced  by  careful  proportioning  of  the 
materials.     These  are  the  best  of  all  surfaces  on  concrete,  and 
are  obtained  by  using  a  medium  wet  mixture  and  carefully 
spading  (p.  189)  so  as  to  get  a  rich  mixture  in  contact  with  the 
form.     Then,  if  the  forms  are  removed  with  sufficient  care,  the 
surface  of  the  material  will  be  such  that  it  cannot  be  much 
improved  by  later  treatment,  particularly  if  the  concrete  has 
been  properly  proportioned  in  the  first  place. 

(2)  Porous  surfaces  caused  by  the  leanness  or  dryness  of  the 
mixture,  by  bad  mixing  or  insufficient  tamping,  by  adultera- 
tion or  careless  working.     The  chief  cause  is  attempting  to 
reduce  the  cost  of  working. 

(3)  Crocodile  surfaces  due  to  the  cement  "  floating  "  to  the 
surface,    then   shrinking   and   cracking,    or    "  alligating  "    on 
account  of  the  difference  in  the  contraction  of  the  cement  and 
concrete.     In  bad  cases  the  surface  spalls  or  peels  away,  but 
usually  it  is  covered  with  minute  hair-lines,   which  bear  a 
fancied  resemblance  to  a  crocodile  skin.     This  surface  is  usually 
due  to  working  with  too  wet  a  mixture.     Surfaces  coated  with 
neat  cement  often  produce  these  hair-lines.      Such   surfaces 
and  lines  are  always  due  to  excessive  shrinkage. 

The  treatment  of  the  surface  of  concrete  is  conveniently 
divided  into  four  main  groups  : — 

(1)  Cleaning  the  surface  so  as  to  remove  dirt,  irregularities, 
etc.  This  may  be  effected  by  brushing,  chipping,  rubbing 
(with  carborundum  bricks),  sand-blasting,  etc.,  or  by  washing 
with  soap  and  water,  acetic  or  hydrochloric  acid  and  water,  or 
plain  water.  Efflorescence  or  scum  is  usually  removed  by 

02 


196     THE  PREPARATION  OF  CONCRETE 

washing  with  weak  hydrochloric  acid,  followed  by  plenty  of 
water  applied  by  means  of  a  hose.  If  the  surface  is  to  be 
cleaned  by  brushing  it  is  usually  necessary  to  do  so  whilst 
the  concrete  is  still  green1,  as  otherwise  the  process  would  be 
too  laborious.  Hence,  the  forms  must  be  removed  within 
twenty-four  hours  of  placing  the  concrete.  This  .limits  the 
applicability  of  simple  brushing. 

Hard  concrete  may  be  cleaned  with  acid  or  with  a  bush- 
hammer  or  a  pneumatic  hammer,  but  the  last-named  removes 
the  cement  surface  and  so  reduces  the  water-proofness  of  the 
concrete — an  objection  which  applies  to  all  methods  of  cleaning 
concrete  surfaces. 

(2)  Filling  in  the  surface   voids,  without  discoloration,  in 
order  to  produce  a  more  pleasing  surface.     Plaster  is  unsuitable 
for  this  purpose  as  it  falls  away  when  the  concrete  becomes 
damp,  and  grout  is  unstable  though  largely  used.     A  1:3:5 
mixture  with  fine  aggregate  is  generally  considered  to  be  one 
of  the  most  suitable  ;  it  is  made  very  thin  and  is  applied  with 
a  whitewash  brush.      The  chief  disadvantage  of  most  fillings 
applied  to  concrete  lies  in  their  great  liability  to  scale  and 
peel  off.     This  does  not  always  commence  at  once,  but  may 
begin  after  two  or  three  years. 

The  only  surface  fillings  which  are  likely  to  be  permanent 
must  contain  a  pigment  which  is  resistant  to  the  action  of 
the  sun  as  well  as  to  damp,  which  is  sufficiently  heavy  to 
fill  the  voids  and  prevent  absorption,  and  of  such  a  nature  that 
the  texture  of  the  finished  surface  bears  a  sufficiently  close 
resemblance  to  that  of  the  original  concrete. 

Ordinary  concrete  cannot  be  given  a  polished  surface 
because  the  particles  of  cement  crystals  are  too  soft  in  propor- 
tion to  the  aggregate,  and  are  easily  reduced  to  powder  without 
securing  a  smooth,  hard  surface  capable  of  reflecting  light. 
Rich  concrete,  in  which  the  aggregate  is  in  the  state  of  a  very 
fine  powder,  placed  in  a  glass-lined  mould  gives  the  nearest 
approach  to  a  polish  at  present  attainable. 

(3)  Treatment    of   the    surface   with    a   coloured   material. 
Specially  prepared  paints  are  sold  for  this  purpose,  but  black 
or  white  marble  and  other  substances  may  be  introduced  in 

1  Concrete  which  has  not  reached  its  greatest  hardness  is  said  to  be  green. 


SURFACE  TREATMENT  OF  CONCRETE    197 

the  material  nearest  to  the  forms.  The  face  is  "  cleaned  up  " 
as  soon  as  possible  after  the  centering  has  been  removed.  If 
necessary,  distance  pieces  may  be  used  in  the  forms  and  the 
surface  concrete  may  in  this  way  be  cast  quite  independently 
of  the  backing. 

Specially  made  cement  mixtures — sometimes  known  as 
granolithic  facings — are  sometimes  poured  in  a  narrow  space 
arranged  between  the  concrete  and  the  form.  There  is, 
however,  considerable  difficulty  in  getting  the  face  to  adhere 
to  the  concrete  without  alligating  or  spalling,  owing  to  the 
difference  in  contraction  between  the  two  mixtures.  The 
facing  mixtures  may  contain  red  granite  or  marble  chippings 
in  order  to  give  a  surface  of  a  colour  different  from  the  natural 
grey  of  concrete.  Other  colouring  materials  are  used  for  the 
same  purpose  ;  they  are  added  in  suitable  proportions  to  the 
cement-sand  mixture  used  for  facing.  Those  most  usually 
employed  are  :  raw  iron  oxide  for  bright  red,  roasted  iron 
oxide  for  brown,  ultramarine  for  bright  blue,  yellow  ochre  for 
buff  to  yellow,  carbon  black  or  lampblack  for  grey  to  dark 
slate,  manganese  dioxide  for  black  (11  Ibs.  per  bag  of  cement). 
A  mixture  of  equal  parts  of  carbon  black  and  red  iron  ore  for 
dull  reds. 

Facings  of  terra  cotta — either  plain  or  glazed — are  also 
extensively  employed,  as  they  give  a  particularly  pleasing 
appearance  and  add  a  warmth  of  tone  not  otherwise  obtainable. 

Most  facings  only  make  concrete  damp-proof,  and  not 
always  that,  as  dampness  may  rise  through  the  foundations 
by  capillary  attraction  in  the  case  of  very  porous  concrete. 
For  this  reason  it  is  generally  better  to  adopt  some  means  of 
rendering  the  whole  mass  waterproof,  and  not  to  rely  too  much 
upon  a  surface-finish.  (See  footnote  on  p.  171.) 

(4)  Treatment  with  a  view  to  preserving  the  concrete  from 
the  action  of  the  weather,  viz.  :— 

(a)  Damp-proofing    with    only   partial    obliteration    of   the 

surface  and  preservation  of  decorative  feature. 

(b)  Waterproofing  with  complete  obliteration  of  the  surface. 
The  term  damp-proofing  should  be  confined  to  methods  and 

appliances  used  for  keeping  water  and  dampness  out  of  the 
superstructures  of  buildings,  the  term  "  waterproofing  "  being 


198     THE  PREPARATION  OF  CONCRETE 

used  for  treating  work  subject  to  hydrostatic  pressure  and  for 
vessels  intended  to  contain  or  retain  water.  Three  distinct 
classes  of  damp-proofing  materials  are  used  :— 

(1)  Transparent  coatings. 

(2)  Opaque  cement  coatings. 

(3)  Special  bituminous  coatings. 

Transparent  coatings  include  those  which  do  not  change  the 
appearance  of  the  surface  treated. 

The  most  frequently  used  materials  are  :  soaps,  oils  and 
various  waxes  together  with  fluates,  water-glass,  casein  paints 
and  bitumens. 

The  old  Sylvester  process,  although  now  practically  obsolete, 
was  one  of  the  earliest  efforts  in  this  kind  of  damp-proofing. 
This  process  involves  alternate  treatments  of  the  surface  with 
solutions  of  soap  and  alum,  and  depends  for  its  efficiency  upon 
the  formation  of  aluminium  salts  of  the  fatty  acid  contained 
in  the  soap,  which  are  insoluble  and  possess  a  very  distinct 
but  temporary  water-repellent  action.  This  process  is  not 
economical,  as  it  is  necessary  to  repeat  the  operations  a  number 
of  times  to  produce  sufficient  insoluble  soap. 

In  the  treatment  of  the  porous  surface  with  hot  paraffin, 
the  exposed  surfaces  are  carefully  heated  and  coated  with 
melted  paraffin  wax  applied  with  a  brush.  By  this  means  the 
paraffin  penetrates  to  a  considerable  depth  before  it  chills  and 
is  thereby  deposited  in  the  pores.  Fairly  successful  results 
can  be  obtained  by  this  method,  but  the  expense  makes  its  use 
quite  prohibitive  in  most  cases. 

Water-glass  has  also  been  applied  with  some  success,  but  it 
is  difficult  to  get  it  into  the  pores  of  the  concrete.  Oxalate  of 
soda  and  various  zinc  compounds  are  also  used. 

Most  of  the  transparent  liquid  coatings  which  are  applied 
to  the  surface  with  a  brush,  like  paint,  and  offered  as  infallible 
remedies  for  dampness  and  porosity,  consist  of  a  paraffin  or  wax 
of  low  melting  point,  dissolved  in  a  light  volatile  oil.  They 
depend  for  their  efficiency  upon  the  deposition  of  the  wax  or 
paraffin  in  the  pores  of  the  concrete.  Some  of  these  water- 
proofings  contain  over  95  per  cent,  of  volatile  constituents  and 
a  very  small  amount  of  solid  base,  only  the  latter  forming  the 
waterproofing  agent. 


DAMP -PROOFING  CONCRETE        199 

Quite  recently,  some  progressive  manufacturers  have  been 
able  to  produce  synthetic  water-repellent  bases  which  form  far 
stronger  solutions  in  volatile  vehicles,  and  these  are  more 
satisfactory. 

Under  the  term  fluates,  various  soluble  silico-fluorides  are 
largely  in  use  for  rendering  concrete  water-proof  and  for 
increasing  the  hardness  of  its  surface.  Aluminium,  magnesium 
and  zinc  silico-fluorides  are  the  ones  chiefly  used  for  this  pur- 
pose, the  concrete  being  either  soaked  in  a  solution  of  fluate 
or  painted  with  the  latter.  These  fluates  act  in  a  manner 
similar  to  water-glass,  but  form  a  harder  product. 

There  is  a  very  distinct  field  for  all  such  coatings,  as  they 
are  the  only  means  available  for  treating  the  exterior  surfaces 
of  porous  stone  and  concrete  in  existing  structures  without 
altering  their  appearance. 

Opaque  Coatings  for  Cement  include  paints  and  coatings  of 
plain  cement  grout,  with  the  attendant  difficulty  of  obtaining 
a  perfect  bond  and  the  tendency  of  the  coating  to  absorb 
water.  Several  manufacturers  have  produced  coatings  made 
with  a  Portland  cement  base,  which  show  a  perfect  bond  on 
the  surface  coated  and  are  perfectly  repellent  and  damp- 
proof.  These  products  contain  no  oil  and  possess  none  of  the 
characteristic  qualities  of  oil  paints.  It  is  well  known  that 
an  oil  paint  must  not  be  applied  directly  to  a  concrete  surface, 
as  the  vegetable  oils  used  react  with  the  alkali  in  the  cement, 
forming  a  soap,  and  cause  the  disintegration  of  the  coating. 
This  may  be  avoided  by  the  employment  of  a  fluate  or  of  a 
coating  of  casein  paint  previous  to  the  use  of  the  oil  paint. 
Oil  paints  also  dry  with  a  distinct  gloss,  which  is  very  ob- 
jectionable on  a  concrete  surface,  where  the  coating  should 
retain  the  characteristic  texture  of  the  surface  coated.  Casein 
paints  followed  by  treatment  with  formaldehyde  are  becoming 
increasingly  popular. 

Where  colour  is  of  no  importance,  as  in  some  underground 
work,  there  is  no  better  paint  than  ordinary  tar,  applied 
hot. 

Cement  coatings  have  a  very  general  application  for  making 
cement  surfaces,  such  as  stucco,  cement  blocks,  etc.,  uniform 
in  colour  and  are  also  used  to  replace  the  somewhat  cheerless 


200     THE  PREPARATION  OF  CONCRETE 

and  unattractive  surfaces  of  untreated  concrete  with  a  soft- 
toned  surface  which  is  thoroughly  damp-proof. 

Special  Bitumens  are  not  applied  to  the  exposed  surface, 
but  to  the  interior  of  concrete  walls.  They  are  black  in 
appearance  and  are  made  of  various  waterproof  gums.  They 
are  applied  with  a  brush,  and,  besides  forming  a  damp-proof 
surface,  they  provide  a  good  bond  for  a  coat  of  plaster  applied 
directly  to  them. 

These  products  eliminate  the  necessity  of  furring  and  lathing, 
and  so  increase  the  available  space  and  remove  all  the  disagree- 
able features  of  the  air  space.  Although  the  film  is  a  fairly 
good  non-conductor,  it  has  not  the  same  insulating  efficiency 
as  an  air  space,  and  is  not  recommended  where  there  is  serious 
condensation  on  the  inner  surface. 

The  term  waterproofing  should  be  confined  to  treating 
structures  subject  to  water  pressure  and  those  designed  to 
retain  water,  but  not  to  prevent  mere  dampness.  One  of  the 
two  following  methods  is  generally  employed  : — 

(1)  "  Integral,"  or  rigid  method  in  which  a  waterproofing 
compound  is  incorporated  in  the  concrete  mass,  rendering  the 
same  waterproof  within  itself. 

(2)  "  Membrane,"  or  bituminous  shield  method,  in  which 
the  concrete  work  is  insulated  from  contact  with  the  water  by 
interposing  a  continuous,  waterproof,  bituminous  shield. 

The  "  Integral  "  method  involves  the  addition  of  a  compound 
to  the  composition  of  the  concrete  during  the  mixing  or 
placing,  and  this  compound  thus  becomes  an  integral  part  of 
the  mass  of  substances.  Two  classes,  characterised  by  the 
physical  condition  in  which  they  are  added  to  the  concrete, 
are  used  : — 

(1)  Finely   powdered    dry   compounds    added    to    the    dry 
cement  in  the  proportion  of  about  2  per  cent. 

(2)  Liquids  or  pastes  added  to  the  water  used  to  temper  the 
dry  mixture  of  cement  and  aggregate. 

The  compounds  in  the  first  class  are  usually  hydrated  lime 
with  a  greater  or  less  amount  of  the  lime  salts  of  fatty  acids. 

The  first  essential  for  success  lies  in  obtaining  an  even, 
homogeneous  distribution  of  the  waterproofing  compound. 
This  is  very  difficult  on  account  of  these  dry  compounds. 


WATERPROOFING  CONCRETE  201 

No  matter  how  great  an  effort  is  made  to  mix  a  repellent 
compound  with  dry  cement  and  then  with  dampened  sand,  as 
soon  as  the  water  is  added  the  repellent  property  of  the  com- 
pound will  manifest  itself,  and  as  the  fluidity  of  the  concrete 
increases,  and  there  will  be  a  tendency  for  the  repellent  com- 
pound to  be  concentrated  in  other  sections,  thereby  making 
an  even,  homogeneous  distribution  impossible.  Although  the 
repellent  feature  is  an  excellent  property  for  a  compound  to 
possess  when  in  place  in  the  mass  of  concrete,  its  very  nature 
makes  even  distribution  difficult,  and  thereby  defeats  its 
intended  purpose  (see  p.  205). 

In  the  case  of  compounds  that  are  added  directly  to  the  water, 
on  the  contrary,  there  is  no  difficulty  in  obtaining  an  even 
distribution,  as  the  water  acts  as  a  vehicle  or  carrier  and 
evenly  distributes  them.  Hence,  as  far  as  the  homogeneous 
disposition  of  the  waterproofing  agent  is  concerned,  compounds 
that  are  originally  miscible  with  water  have  a  decided  advan- 
tage over  dry  compounds  of  a  repellent  nature.  A  suitable 
compound  for  this  class  of  work  should  not  contain  any  organic 
constituents  or  other  materials  capable  of  interfering  with  the 
strength  of  the  concrete,  as  it  should  not  be  necessary  to 
sacrifice  strength  to  obtain  waterproofing  efficiency.  Yet 
results  have  been  obtained  which  indicate  a  loss  of  over  50  per 
cent,  in  the  strength  of  concrete  when  emulsified  oils  were 
contained  in  the  compound  used  for  waterproofing. 

The  "  Integral "  method  of  waterproofing  has  a  very 
general  application  to  waterproofing  conditions,  though  it 
cannot  be  used  in  cases  when  there  is  a  liability  for  the  con- 
tinual development  of  cracks  in  the  work,  as  these  would,  of 
course,  destroy  the  waterproofing  efficiency.  The  "Integral" 
method  is  very  largely  employed  for  substructural  work, 
cisterns,  reservoirs,  etc.,  which  are  designed  for  containing 
water.  The  waterproofing  compounds  can  be  used  throughout 
the  mass  of  concrete,  or  in  cases  where  this  precedure  would 
be  impracticable,  on  account  of  the  cost,  it  can  be  concentrated 
in  a  plaster  coat  on  the  surface  of  the  structure  (see  p.  205). 

The  Coating  of  waterproofed  cement  mortar  should  be 
prepared  by  thoroughly  tempering  to  required  consistency  a 
dry  mixture  of  one  part  of  cement  to  two  parts  of  sand 


202     THE  PREPARATION  OF  CONCRETE 

with  water,  to  which  the  waterproofing  compound  has  been 
added  in  the  proportion  directed  by  the  manufacturer.  The 
sand  must  be  clean  and  spherical  and  well  graded.  Before 
plastering  such  cement  mortar  on  to  old  concrete,  the  surface 
of  the  latter  should  be  treated  as  follows  :— 

(a)  The  old  surface  must  be  cleaned  very  thoroughly  with 
a  heavy  wire  broom  so  as  to  remove  all  dust  and  dirt.     A  jet 
of    steam    should,    if    available,    be   employed   to   clean   the 
wall. 

(b)  To  the  mechanically-cleaned  surface  a  liberal  coat  of 
1  :  10  solution  of  hydrochloric  acid  is  applied  with  a  large 
brush.     The  acid  remains  until  it  has  exhausted  itself,  which 
will  require  at  least  ten  minutes.     A  second  liberal  coating 
of  acid  solution  should  then  be  applied  before  removing  the 
first,  and  a  third  coat  if  the  two  applications  have  not  satis- 
factorily  exposed   the   aggregate   and   entirely   removed   the 
skin  of  hardened  cement. 

(c)  With  a  hose,  under  good  pressure,  the  surface  should 
be  washed  in  one  direction  so  as  to  remove  the  salts  resulting 
from  the  action  of  the  acid.     This  washing  is  continued  until 
the  salts  and  all  loose  particles  are  removed  and  the  old  concrete 
is  thoroughly  soaked  to  its  full  hydrometric  capacity. 

(d)  To  the  cleaned  and  saturated  surface  a  coating  of  pure 
cement  is  applied,  mixed  to  the  constituency  of  thick  cream 
with  water,  and  to  which   the  waterproofing  agent  has  been 
added  in  the  proportion  directed   by  the    manufacturer,  the 
coating  being  rubbed  in  vigorously  with  a  strong  fibre  brush 
so  as  to  fill  all   the  crevices   and  cavities  produced  by  the 
action  of  the  acid. 

Immediately  after  applying  the  above  slush  coat,  the  first 
coating  of  waterproof  cement  mortar  should  be  applied 
(thickness,  three-eighths  of  an  inch)  directly  upon  the  slush 
coating  and  well  trowelled  into  every  void  or  crevice  of  the 
surface.  Before  this  first  coat  has  reached  its  final  set,  a 
second  and  final  coat  should  be  applied  to  an  equal  thickness, 
so  as  to  make  the  full  average  thickness  three-quarters  of  an 
inch.  The  finishing  coat  should  be  floated  to  an  even  surface, 
and  subsequently  trowelled  free  from  any  porous  imperfections. 
If  the  conditions  of  the  work  make  it  impracticable  to  apply 


WATERPROOFING  CONCRETE  203 

a  finishing  coat  before  the  scratch  coat  has  set,  the  latter 
must  be  dampened  and  slush  coated  before  the  finishing 
coat  is  applied. 

Floors  should  be  treated  and  prepared  exactly  as  indicated 
above  for  walls,  and  finished  with  the  waterproofed  mortar 
to  a  thickness  of  two  inches.  Special  care  should  be  exercised 
to  bond  the  wall  coating  to  the  floor  coating,  so  as  to  make 
the  waterproof  coat  absolutely  continuous. 

The  "  Membrane  "  method  of  waterproofing  differs  distinctly 
from  the  "Integral,"  in  that  it  does  not  attempt  to  treat  the 
concrete,  but  rather  to  insulate  it  from  contact  with  water 
by  enveloping  the  structure  in  a  continuous  bituminous  shield. 
The  fact  that  the  "  Membrane  "  is  not  a  unit  or  rigid  part  of 
the  structure  permits  a  certain  freedom  of  movement  and 
action  in  the  structure,  without  impairing  the  efficiency  of  the 
waterproofing.  This  feature  of  the  "  Membrane  "  system 
makes  it  specially  suitable  for  waterproofing  work  not  fully 
reinforced  and  liable  to  settlement  or  subject  to  vibration  or 
shock,  such  as  railway  bridges,  culverts,  etc. 

Coatings  of  burlap  and  coal-tar  felts  have  been  extensively 
used  for  this  purpose,  and  there  are  now  on  the  market  specially 
manufactured  felts  which  are  both  saturated  and  coated  with 
bitumen,  and  possess  great  pliability  and  strength. 

The  bitumens  most  generally  used  for  cementing  the  felt 
together  in  constructing  the  membranes  are  coal-tar  pitch, 
commercial  asphalts  and  special  asphalt  compositions. 
Although  coal-tar  pitch,  on  account  of  its  cheapness,  has  been, 
and  still  is,  being  very  extensively  employed  for  waterproofing, 
many  engineers  regard  the  coal-tar  pitch  produced  to-day  by 
modern  methods  of  gas  production  as  inferior  to  that  produced 
by  the  older  process  in  vogue  when  pitch  was  first  used  -  for 
waterproofing. 

The  asphalts  are  more  suitable  for  waterproofing  as  they 
possess  greater  elasticity  and  permanence,  but  care  should 
be  taken  to  obtain  a  material  of  as  low  a  melting  point  as  the 
nature  of  the  work  will  permit. 

This  not  only  ensures  greater  elasticity  when  subjected  to 
cold  temperatures,  but  it  is  much  more  freely  and  easily 
applied.  Special  asphalts  are  manufactured  from  a  hard 


204  THE  PREPARATION  OF  CONCRETE 

hydrocarbon,    such    as    gilsonite,  tempered    with    petroleum 
residuums  to  impart  the  necessary  elasticity. 

Oils, — Animal  and  vegetable  oils  which  are  liable  to  turn 
rancid  have  a  corrosive  action  on  cement  and  should  not  be 
kept  in  concrete  tanks.  Care  should  also  be  taken  not  to 
spill  such  oils  on  foundations  or  floors  made  of  concrete,  as 
the  acid  produced  when  the  oil  turns  rancid  will  decompose 
the  cement.  Mineral  oils  are  free  from  this  objection. 

The  waterproofing  materials  used  for  concrete  are  of  two 
kinds  :  (a)  those  which  are  merely  pore-fillers,  and  (b)  those 
which  fill  the  pores  and  also  have  a  repellent  action  towards 
water.  Amongst  the  best  pore-fillers  are  rock  dust,  slaked 
lime,  and  china  clay  ;  their  particles  are  so  fine  that  they 
penetrate  almost  all  the  pores,  and  as  they  are  not  greasy  they 
become  uniformly  distributed  throughout  the  material. 

Oils  and  soaps,  on  the  contrary,  tend  to  accumulate  in  small 
pasty  mas^s  which  do  not  readily  break  up,  and  render  it 
almost  impossible  to  obtain  homogeneous  concrete.  To  this 
extent  they  are  a  distinct  disadvantage,  and  many  of  the 
claims  made  for  them  are  more  in  the  nature  of  "  selling 
arguments  "  than  facts  of  technical  importance.  Moreover, 
the  addition  of  oil  or  soap  to  a  concrete  mixture  greatly  delays 
its  setting,  and  does  not  confer  any  advantage  adequate  to 
the  risk  thereby  involved.  Soluble  soaps,  and  particularly 
soft  soap  (potash  soap),  are  better  waterproofing  agents  than 
oils,  because  the  soap  may  be  dissolved  in  the  water  used  and 
so  becomes  more  evenly  distributed  throughout  the  concrete. 
Useful  proportions  are  6  to  9  Ibs.  of  soap  per  cubic  yard  of 
concrete.  The  lime  set  free  by  the  hydrolysis  of  the  cement 
forms  the  insoluble  lime  soap,  and  this  is  the  compound  which 
fills  the  pores  and  repels  water. 

Waterproofing  concrete  is  best  effected  by  rendering  the 
whole  mass  as  impermeable  as  possible,  and  not  merely  con- 
fining the  treatment  to  the  surface.  The  power  of  water  to 
penetrate  concrete  depends  on  the  number  and  size  of  the 
pores  in  the  latter  ;  hence  a  concrete  in  which  all  these  pores 
or  voids  are  completely  filled  with  inert  material  or  are  partially 
filled  with  a  water-resisting  substance  will  be  waterproof. 

If  the  necessary  care  and  skill  have  been  taken  in  preparing 


WATERPROOFING  CONCRETE  205 

it  from  properly  graded  materials,  the  ordinary  concrete  will 
be  sufficiently  waterproof  for  most  purposes.  The  object  of 
water-repellents  is  simply  to  occupy  voids  in  the  concrete  which 
are  produced  by  imperfect  grading  or  mixing  of  the  concrete. 

Most  of  the  substances  sold  for  waterproofing  concrete  are 
of  a  soapy  or  oleaginous  nature,  and  water-glass  (a  soluble 
sodium  silicate)  is  also  used,  though  it  is  costly. 

It  should  never  be  forgotten  that  the  addition  of  oil  or  soap 
or  any  non-cementitious  material  to  concrete  reduces  the 
strength  of  the  structure,  and  is  to  this  extent  disadvantageous. 
Some  oils  reduce  the  strength  much  more  than  is  commonly 
supposed. 

One  of  the  most  effective  methods  of  preparing  a  waterproof 
concrete  consists  in  carefully  grading  and  proportioning  the 
materials  so  as  to  have  as  few  voids  as  possible,  and  to  mix 
them  together  in  a  dry  state.  Then,  instead  of  some  sand 
there  is  used  a  mixture  of  trass  or  burned  clay  in  such 
proportions  that  the  amount  added  will  be  approximately 
half  that  of  the  cement  used.  The  trass  or  burned  clay  powder 
acts  as  a  pozzolana,  fixes  the  lime  set  free  by  the  cement  and, 
if  the  grading  and  proportioning  have  been  properly  effected, 
renders  the  concrete  quite  waterproof.  The  substitution  of  a 
mixture  of  equal  parts  of  china  clay  and  lime,  though  some- 
times recommended,  is  far  less  effective. 

Cloyd  M.  Chapman  has  found  that  in  many  concretes  the 
permeability  to  water  is  due  to  the  use  of  a  mixture  of  aggregate, 
sand,  cement  and  water,  which  is  either  too  dry  or  too  wet — 
one  extreme  being  as  bad  as  the  other  so  far  as  water-proofness 
is  concerned.  His  investigations  showed  that  the  most 
waterproof  concretes  he  was  able  to  produce  were  those  in 
which  the  water  represents  13  to  17  per  cent,  of  the  weight  of 
the  mixture,  but  these  figures  may  differ  with  different  cements 
and  aggregates. 


CHAPTER  VIII 

REINFORCED    CONCRETE 

THE  term  "  Reinforced  Concrete  "  or  "  Ferro-Concrete  "  l  is 
applied  to  structures  consisting  of  a  combination  of  concrete 
and  metal  (usually  steel,  and  termed  the  "  reinforcement  ")  of 
such  a  nature  that  the  two  materials  act  as  one,  which  is 
stronger  and  more  durable  than  either  alone.  Concrete  is  not 
very  suitable  for  withstanding  tensional  stresses  ;  steel,  on 
the  contrary,  is  not  sufficiently  cheap  to  be  used  alone.  Steel 
alone  is  not  very  resistant  to  weather,  and  its  surface  must 
be  protected.  Concrete  preserves  the  steel  from  deterioration 
and,  in  case  of  fire,  from  expansion.  A  combination  of  steel 
and  concrete  is  therefore  capable  of  meeting  the  demands  for 
structures  which  will  resist  both  these  stresses.  In  short, 
reinforced  concrete  combines  the  structural  qualities  of  steel 
and  timber  with  the  durability  of  good  masonry.  It  is  subject 
to  no  form  of  deterioration  which  cannot  be  avoided  by 
reasonable  precautions,  and  is  free  from  many  of  the  limitations 
of  masonry  in  mass.  Because  of  the  greater  latitude  it  affords 
in  the  design  and  execution  of  structures,  it  often  yields  the 
best  and  most  economical  solution,  and  in  some  cases  the  only 
practicable  solution,  of  the  most  difficult  problems  of  building 
construction. 

The  great  advantage  of  reinforced  concrete  lies  in  the  fact 
that  it  is  capable  of  withstanding  stresses  due  to  transverse 
strains,  tension,  and  shearing.  All  the  forms  that  could  be 
executed  in  steel  or  timber  can  be  closely  imitated  in  reinforced 
concrete,  which  is  immune  from  corrosion  and  decay.  This 

1  The  student  should  remember  that  the  terms  "  Ferrocrete  "  and  "Steelcrete" 
have  been  registered  for  a  Portland  cement  and  do  not  refer  to  any  form  of 
reinforced  concrete. 


STEEL  IN  REINFORCED  CONCRETE  207 

makes  it  possible  to  adopt  designs  wherein  the  structure  acts 
by  its  structural  resistance  and  not  by  dead  weight,  and  even 
the  material  to  be  retained  and  held  back  may  be  made  by 
this  means  to  add  to  the  stability  of  the  work  as  a  whole. 
Dead  weights  on  foundations  are  diminished,  difficult  excava- 
tion is  often  avoided  or  lessened,  and  total  costs  often 
greatly  decreased,  as  compared  with  structures  formed  of 
masonry  in  mass ;  in  many  cases  reinforced  concrete  affords 
a  variety  of  desirable  solutions  not  practicable  in  any  other 
material. 

The  saving  in  the  thickness  of  inverts  of  locks  and  dams,  or 
in  retaining  walls  of  all  kinds,  the  use  of  caissons  filled  with 
dead  materials  in  lieu  of  solid  masonry  walls,  the  use  of  rein- 
forced concrete  piles  to  anchor  a  light  structure  to  the  dead 
mass  below,  and  the  many  other  useful  devices  and  applica- 
tions, all  open  up  the  possibility  of  practically  limitless 
applications  of  reinforced  concrete  to  hydraulic  structures 
so  as  to  attain  both  greater  efficiency  and  a  diminished 
cost. 

Unlike  timber,  iron,  and  steel,  which  rapidly  perish  by 
natural  decay  or  corrosion,  and  unlike  many  stones,  which, 
more  slowly,  but  none  the  less  surely,  disintegrate  and  crumble 
away  as  the  result  of  atmospheric  influences,  good  concrete 
increases  in  durability  and  strength  by  continued  exposure. 
The  same  property  is  still  more  highly  developed  in  reinforced 
concrete,  owing  to  the  superior  quality  and  the  scientific 
proportions  of  the  constituent  materials,  and  in  consequence 
of  the  close  attention  devoted  to  the  thorough  mixing  of  the 
ingredients,  and  the  methods  of  depositing  and  tamping  the 
resulting  concrete,  which  are  ensured  when  the  execution 
of  works  is  confined  to  recognised  contractors  whose 
experience  and  standing  give  assurance  of  their  perfect 
reliability. 

When  properly  designed  and  executed  it  is,  therefore,  among 
the  most  valuable  materials  available  for  use  in  structural 
and  hydraulic  works. 

The  chief  objection  to  reinforced  concrete  is  an  aesthetic 
one ;  the  appearance  of  the  finished  work  is  dull  and 
colourless,  without  any  of  the  distinctive  tones  and  colour 


208  REINFORCED  CONCRETE 

associated  with  brick  and  stone  ;  yet,  when  colour  is  of 
minor  importance,  as  in  piers,  jetties,  foundations,  etc., 
reinforced  concrete  has  claims  far  in  advance  of  any  other 
material. 

Steel,  when  under  compression,  is  about  thirty  times  as 
strong  as  concrete,  and  when  under  tension  is  about 
300  times  as  strong  as  concrete.  As  steel  costs  about 
fifty  times  as  much  as  an  equal  volume  of  concrete,  it 
is  possible  to  use  a  combination  of  the  two  which  is 
cheaper  (considering  the  stresses  to  be  resisted)  than  either 
material  alone. 

Hence,  if  the  members  of  the  structures  are  arranged  in 
such  a  way  that  the  compressive  stresses  are  all  borne  by  the 

concrete      and      the 

>^F17T?^*&tftW'^  tensile  stresses  by  the 

£•'      steel,    each    material 

b  will  be  used  for  the 

''^#W%&ty£:>^  -  fvij t'**$$p    purpose  for  which  it 

yi/iS^^i'^  is  the   cheapest   and 

^ 1L''.  .»'•'».'*.•.  '•'•. ••* •' '•'.- •-•"*.. ''.*;'-.' -I.:*  '•'  <-'  $••'/.  °'-VI;-.VA  *••'•••  '*•.'•>': '*•  »-..•.- IrX?         ii T j     .Cj.j.^J]  rpv  „ 


>^r -ty     the  best  fitted.     The 

.  shearing  resistance  of 

l|      |      |      |     |0  |l  \3  \*  y 

SCAU-FECT  concrete  is  also  very 

FIG.  45.— Beams,    of  equal   strength,  of      inar|pnilptp        onrl 
(a)  Keinforcedand  (b)  Ordinary  Concrete,      inadequate,      ana 

hence,   in    the    most 

approved   designs,   the   shearing  stresses  are  borne  by  small 
steel  rods  and  stirrups  embedded  in  the  concrete. 

The  remarkable  effect  of  steel  in  increasing  the  strength  of 
concrete  is  strikingly  illustrated  in  Fig.  45,  which  shows  two 
beams  designed  to  carry  ordinary  floor  loads,  the  one  made 
entirely  of  concrete  and  the  other  of  concrete  with  a  sheet  of 
expanded  metal  embedded  in  the  tensile  portion  of  the  beam. 
The  saving  in  mere  weight  of  concrete  alone  is  obvious  ;  and 
when  it  is  remembered  that  the  adoption  of  floor  beams  entirely 
of  concrete  means  an  increase  in  thickness  of  nine  inches,  or 
supposing  five  to  eight  floors,  an  increase  in  the  total  height 
of  the  building  (with  extra  cost  of  higher  and  heavier  walls, 
together  with  heavier  foundations  to  carry  them)  of  from 
four  to  six  feet,  it  is  clear  that  even  as  regards  initial  outlay 
for  materials,  the  introduction  of  steel  reinforcement  into 


AGGREGATES  FOR  REINFORCED  CONCRETE     209 

concrete  construction  is  of  very  great  importance.  Another 
most  remarkable  fact  is  that  the  weight  of  steel,  if  properly 
disposed,  is  so  small  as  almost  to  be  insignificant.  Comparing 
areas  of  steel  and  concrete  exposed  in  cross-section,  the  steel 
is  sometimes  only  J  per  cent.,  and  rarely  rises  above  1  or  1 J  per 
cent,  of  the  area  of  the  concrete. 

The  concrete  must  be  of  first-class  quality,  and  the  aggregate 
must  be  smaller  than  that  used  for  mass  concrete.  No  pieces 
of  .aggregate  which  will  not  pass  through  a  hole  IJ  inches 
diameter  should  be  used. 

Both  the  aggregate  and  sand  must  be  carefully  selected, 
especially  when  a  combination  of  strength  and  fire  resistance 
is  desired.  A  hard  aggregate,  such  as  river  ballast,  pit  gravel, 
blue  bricks,  granite,  etc.,  is  preferable,  as  a  soft  one,  such  as 
clinker,  red  brick,  sandstone,  etc.,  means  a  weaker  concrete. 
The  stones  should  be  angular  and  of  an  irregular  nature,  both 
in  shape  and  size.  Flaky  aggregates  should  be  avoided  as 
they  lie  too  close  together. 

For  encasing  steelwork,  flooring  and  similar  work,  brick  or 
well-burned  furnace  clinker,  or  a  mixture  of  the  two,  affords 
an  excellent  fire-resisting  aggregate,  but  must  be  hard  and  free 
from  combustible  materials,  old  mortar,  ashes  and  dust. 

The  natural  aggregates,  while  they  have  greater  compres- 
sional  strength,  are  liable  to  splinter  and  "  fly  "  under  intense 
heat  ;  limestone  will  shrink  when  exposed  to  fierce  fire,  and 
sandstone  is  so  variable  when  heated  that  it  should  be  avoided 
where  fire  resistance  is  required. 

Cinders  and  coke  breeze  are  not  recommended  for  reinforced 
concrete  work,  and  their  use  should  be  restricted  to  filling 
purposes,  such  as  bedding  to  which  to  nail  floor  boards. 

The  sand  used  should  be  hard  and  gritty,  with  grains  of 
various  shapes  and  sizes.  Sharp,  angular  sand  is  preferable 
for  reinforced  concrete  work.  Crushed  limestone  should  be 
avoided,  and  clayey  sand  should  be  entirely  prohibited  unless 
the  clay  is  first  removed  by  very  thorough  washing.  Sea 
sand  sometimes  causes  an  efflorescence  or  scum  to  appear  on 
the  finished  work.  Very  fine  or  "  blown  "  sand  should  not 
be  used. 

Both  the  aggregate  and  the  sand  should  be  clean,  sharp  and 

c,  p 


210  REINFORCED  CONCRETE 

free  from  all  foreign  substances,  and,  if  necessary,  should  be 
washed  before  use. 

Materials  containing  more  than,  say,  1  per  cent,  of  sulphates 
or  other  corrosive  substances,  should  be  strictly  avoided,  and 
when  any  doubt  exists  on  this  point  it  is  wise  to  have  a  chemical 
analysis  of  them  made  to  ensure  the  absence  of  such  ingredients. 
Some  bricks — notably  some  made  at  Fletton,  near  Peter- 
borough— have  proved  unsatisfactory  as  aggregates  on  account 
of  the  sulphur  compounds  they  contain. 

The  cement  employed  must  be  Portland  cement  which 
conforms  to  the  Standard  Specification  (p.  97),  as  no  other  is 
of  sufficiently  good  quality.  The  risks  in  defective  reinforced 
concrete  are  so  enormous  that  no  pains  should  be  spared  to 
prevent  them. 

The  following  mixtures,  all  parts  by  measure,  are  typical  of 
those  used  by  the  most  reliable  firms  :— 

For  floors,  walls,  etc.  : — 

( |  to  3 1  inches  thick)  (thicker  than  3  J  inches) 

3  parts  of  aggregate.  4  parts  of  aggregate. 

If  parts  of  sand.  2  parts  of  sand. 

1  part  of  Portland  cement.  1  part  of  Portland  cement. 

For  general  and  heavy  concrete  work  :— 

3  parts  of  aggregate.  \      f4  parts  of  aggregate. 

2J  parts  of  sand.  [or|  2  parts  of  sand. 

1  part  of  Portland  cement.)      (l  part  of  Portland  cement. 

For  tanks,  etc.,  where  the  concrete  is  required  to  resist  liquid 
pressures  : — 

3  parts  of  aggregate. 

2  parts  of  sand. 

1  part  of  Portland  cement. 

The  practice  of  using  fixed  proportions  of  aggregates,  sand 
and  cement  is  particularly  unsatisfactory  and  dangerous  ;  in 
every  case  the  proportions  should  be  selected  after  ascertaining 
the  percentage  of  voids  in  the  materials  as  described  on  p.  157. 

The  mild  steel  used  does  not  vary  in  quality  so  much  as  the 
concrete.  In  tension  the  strains  are  proportional  to  the  stress 
below  the  elastic  limit,  beyond  which  it  is  unsafe  to  stretch  it. 


STEEL  FOR  REINFORCEMENT  211 

The  mild  steel  used  for  reinforcement  usually  has  an  elastic 
limit  corresponding  to  a  stress  of  32,000  to  50,000  Ibs.  per 
square  inch.  The  working  stress  specified  in  reinforced  concrete 
is  usually  about  half  this,  namely,  16,000  to  25,000  Ibs.  per 
square  inch.  The  working  stress  in  steel  beams  under  com- 
pression may  be  taken  at  16,000  Ibs.  per  square  inch,  and  for 
columns  at  12,000  Ibs.  per  square  inch,  or  still  lower  if  there  is 
any  probability  of  buckling. 

The  coefficient  of  expansion  of  steel  and  concrete  are  almost 
identical  at  ordinary  temperatures,  otherwise  serious  internal 
strains  would  be  produced.  The  actual  coefficients  of  expan- 
sion vary  with  different  specimens,  but  on  an  average  steel 
expands  -00065  per  cent.,  and  the  concrete  -00060  per  cent, 
for  a  rise  in  temperature  of  each  degree  Fahrenheit. 

Steel  with  a  high  percentage  of  carbon  is  unsuitable  for  use 
in  reinforced  concrete,  as  it  is  brittle  (particularly  after  ham- 
mering), is  more  costly  and  more  difficult  to  work  than  mild 
steel.  Steel  containing  less  than  0-3  per  cent,  of  carbon  is 
not  open  to  this  objection,  and  consequently  mild  steel  is 
almost  invariably  employed. 

The  Committee  of  the  Concrete  Institute  recommended 
that  the  steel  used  shall  have  the  following  properties  : — 

(a)  It  shall  attain   an  ultimate  tensile  strength  of  not  less  than 
60,000  Ibs.  per  square  inch. 

(b)  It  shall  withstand  a  stress  of  at  least  34, 000  Ibs.  per  square  inch 
before  showing  any  appreciable  permanent  set. 

(c)  The  contraction  of  area  at  fracture  shall  be  not  less  than  45  per 
cent.,  or  the  elongation  in  the  case  of  bars  of  one  inch  diameter  and 
under  shall  be  not  less  than  25  per  cent.,  measured  on  a  length  equal  to 
eight  times  the  diameter  of  the  bar  tested. 

The  elongation  shall  be  measured  in  the  case  of  bars  over  one  inch 
diameter  on  a  length  equal  to  four  diameters  of  the  bar,  and  shall  be 
not  less  than  30  per  cent. 

(d)  All  steel  shall  stand  bending  cold  to  an  angle  of  180  degrees 
around  a  diameter  equal  to  that  of  the  piece  tested,  without  fracturing 
the  skin  of  the  bent  portion. 

(e)  The  steel  shall  be  free  from  scabs  and  flaws,  and  must  be  clean 
and  free  from  rust.     It  must  not  be  painted  or  oiled,  but  a  wash  of 
Portland  cement  grout  is  desirable. 

The  Joint  Committee  under  the  auspices  of  the  Royal 
Institute  of  British  Architects  has  made  very  similar  recom- 
mendations, but  in  (b)  this  committee  places  the  elastic  limit 
at  not  less  than  10  per  cent,  nor  more  thun  60  per  cent,  of  the 

P  2 


212  REINFORCED  CONCRETE 

ultimate  strength,  and  the  minimum  elongation  (c)  at  22  per 
cent.,  and  requires  the  steel  to  stand  the  other  tests  specified 
in  the  British  Standard  Specification  for  Structural  Steel. 

Welding  is  to  be  avoided  wherever  possible  ;  if  necessary,  it 
should  be  at  the  points  of  minimum  stress. 

Some  of  the  firms  specialising  in  concrete  construction  have 
even  stricter  conditions  in  their  specifications.  The  following 
extracts  from  specifications  issued  by  several  leading  firms 
give  some  idea  of  these  additional  requirements  :— 

British  Reinforced  Concrete  Engineering  Co.,  Ltd. 

All  rods,  plates,  bars  or  braces  to  be  of  mild  steel,  manufactured  on 
open-hearth  basic  or  acid  Siemen's  process,  uniform  in  quality,  and 
entirely  free  from  defects. 

Ultimate  tensile  strength  not  less  than  28  nor  more  than  32  tons  per 
square  inch. 

All  steel  on  delivery  to  be  cleaned  and  stored  in  a  dry  place. 

All  stirrups  and  hoops  to  accurately  fit  rods  and  bars  to  be  bent  to 
proper  shape. 

Trussed  Concrete  Steel  Co.,  Ltd.  (Kahn  System). 

No  reinforcing  steel  shall  be  considered  that  does  not  provide  for 
shearing  stresses  as  well  as  direct  tension. 

These  shear -resisting  members  must  be  inclined  at  an  angle  of 
45  degrees,  pointing  up  and  towards  the  supports  of  the  structure. 

Shear  members  shall  be  rigidly  attached  to  main  tension  members. 

Sufficient  steel  to  be  placed  that  concrete  shall  be  obliged  to  resist 
only  direct  compression  and  shearing  stresses  up  to  50  Ibs.  per  square 
inch. 

No  steel  shall  have  at  any  point  less  than  one  inch  concrete  covering. 

In  no  case  will  steel  of  a  higher  elastic  limit  than  45,000  Ibs.  be 
considered.  Same  shall  have  a  tensile  strength  of  from  60,000  to 
70,000  Ibs.  per  square  inch,  with  elongation  not  less  than  20  per  cent, 
in  eight  inches. 

British  Concrete- Steel  Co. 

The  indented  steel  bars  to  be  of  best  quality  :  tensile  strength 
38  to  42  tons  per  square  inch.  Elastic  limit  not  less  than  50,000  Ibs. 
(22  tons)  per  square  inch. 

Ample  supply  of  soft  iron  wire  is  to  be  provided  for  lapping  steel 
bars  at  joints  and  at  points  where  they  cross  each  other. 

L.  0.  Mouchel  and  Partners  (Hennebique  System). 

Steel  to  be  in  the  form  of  round  bars  and  strip,  obtained  from  makers 
of  good  repute,  and  to  be  mild  steel  produced  by  the  open  hearth,  basic 
or  acid  process.  Neither  Bessemer  steel  nor  high  carbon-steel  to  be 
employed. 

Consider e  Construction  Co.,  Ltd. 

Steel  must  be  of  British  manufacture  and  have  tensile  strength  not 
less  than  28  tons  or  more  than  32  tons  per  square  inch,  and  show 


COMMERCIAL  SPECIFICATIONS  213 

contraction  of  area  at  fracture  of  50  per  cent,  and  40  per  cent,  respec- 
tively, and  appearance  of  fracture  not  to  show  more  than  5  per  cent, 
and  10  per  cent,  granular  surface  respectively. 

Expanded  Metal  Co.,  Ltd. 

Mixture  of  concrete  not  less  than  1:2:4  by  volume.  Working 
stress  not  exceeding  16,000  Ibs.  per  square  inch  in  tension  in  the 
expanded  steel,  and  500  Ibs.  per  square  inch  extreme  surface  com- 
pression in  the  concrete.  Ratio  of  the  moduli  of  elasticity  of  steel  and 
concrete  taken  as  15. 

Edmond  Coignet,  Ltd. 

Annealed  wire  used  for  binding  together  various  bars  of  framework 
at  intersection  should  be  about  ^-inch  diameter. 

Binding  to  be  done  as  tightly  as  possible  and  cutting  pliers  used. 

It  is  of  the  greatest  importance  that  concrete  structures 
should  not  be  overloaded,  especially  when  their  design  is  such 
that  a  rigid  economy  in  material  has  been  anticipated. 

Loads  in  buildings  and  other  structures  are  of  two  kinds  : 
dead  loads  which  are  constant,  and  live  loads  which  are  moved 
from  time  to  time,  or  may  even  be  of  a  purely  momentary 
character,  such  as  a  train  passing  at  a  high  speed  over  a  bridge. 
Live  loads  usually  tend  to  cause  vibrations  in  the  structure, 
and  consequently  their  effect  is  greater  than  their  actual  weight. 
This  effect  is  most  conveniently  expressed  in  the  form  of  a 
ratio  of  which  the  equivalent  dead  load  forms  one  term. 
Thus  a  floor  intended  to  carry  a  crowd  of  people  would  have 
an  equivalent  dead  weight  of  120  Ibs.  per  square  foot,  whilst 
that  of  a  train  travelling  over  a  bridge  would  correspond  to 
a  dead  load  of  500  Ibs.  per  square  foot. 

When  calculating  loads,  the  weight  of  the  structure  and  all 
fixed  loads  and  the  equivalent  of  any  thrusts  and  other 
forces  must  be  included  in  the  dead  load,  the  weight  of  rein- 
forced concrete  being  usually  taken  at  150  Ibs.  per  cubic  foot. 

The  following  working  stresses  represent  those  commonly 
employed  : — 

Ibs.  per  square  inch 
Steel  in  tension  .          .  16,000 


Steel  in  compression 

Steel  in  shear        .... 

Concrete  in  compression  (bending) 

Concrete  in  compression  (columns,  etc.) 

Concrete  in  shear 

Adhesion  of  concrete  to  steel 


12,000 

8,000 

600 

500 

60 

100 


214  REINFORCED  CONCRETE 

If  the  concrete  has  a  crushing  strength  above  2,400  Ibs.  per 
square  inch  after  twenty-eight  days,  the  working  stress  in 
compression  for  beams  may  be  taken  as  one-fourth,  and  for 
columns,  etc.  as  one-fifth,  of  its  crushing  strength.  It  is  only 
fair  to  point  out,  however,  that  it  is  the  elastic  limit  of  the 
steel  and  not  its  ultimate  strength  which  forms  the  critical 
factor,  as  at  the  yielding  point  of  the  steel  the  whole  member 
will  fail  on  account  of  the  concrete  being  unable  to  stretch  as 
much  as  the  steel  has  done. 

The  following  figures  represent  the  value  ordinarily  assumed 
for  equivalent  dead  loads  on  floors  :— 

Ibs.  per  square  foot. 

Crowd  of  people        .....  120 

Dwelling-houses,  hotels,  etc.       .          .          .          80  to  120 
Theatres,  churches,  etc.     ....        100    ,   150 


Drill  halls  and  ballrooms  .          .          .  .140 

Stores,  warehouses,  and  light  factories  .        100 

Heavy  factories  and  workshops           .  .        200 

Roofs      .  30 


160 

200 

400 

50 


Factors  of  Safety. — Before  the  design  of  a  concrete  structure 
is  prepared,  the  following  particulars  should  be  definitely 
settled  :  (a)  the  live  or  maximum  load  per  unit,  which  should 
not  exceed  one-half  the  elastic  limit  of  the  steel  used  for 
reinforcement  ;  (b)  the  factor  of  safety  for  the  live  load,  which 
is  usually  taken  as  4,  but  sometimes  as  5  or  6  ;  (c)  the  ratio  of 
live  to  dead  load,  usually  taken  as  2  :  1  ;  (d)  the  factor  of  safety 
for  the  dead  load,  usually  assumed  as  three-quarters  of  that 
of  the  live  load  ;  (e)  the  test  load,  which  is  usually  one  and  a 
half  times  the  live  load. 

Where  the  factor  of  safety  is  stated  in  relation  to  the  tensile 
strength  of  the  material  the  elastic  limit  of  the  steel  should 
be  doubled.  For  example,  with  a  steel  whose  elastic  limit  is 
one-half  its  tensile  strength,  the  maximum  live  load  would 
be  one- quarter  of  the  load  representing  its  tensile  strength, 
and  if  4  is  taken  as  the  factor  of  safety,  the  greatest  live 
load  permissible  would  be  one-quarter  of  the  factor  of  safety 
multiplied  by  the  figure  obtained  for  the  live  load. 

The  safe  load  for  Portland  cement  concrete  made  in  the 
usual  proportions  is  from  6  to  8  tons  per  foot  super. 

For  grey  stone  lime  concrete,  it  is  1  to  2  tons. 

For  blue  lias  lime  concrete,  it  is  2  to  3  tons. 


FACTOR  OF  SAFETY 


215 


To  obtain  satisfactory  results  without  using  an  unnecessary 
quantity  of  material,  the  bending  moments,  shearing  forces, 
and  other  stresses  to  which  the  structure  will  be  subjected 
must  be  calculated.  These  calculations  must  be  studied  from 
a  text-book  on  "  The  Theory  of  Structures,"  or  "  Strength  of 
Materials,"  as  they  are  entirely  a  matter  for  structural  engineers 
and  are  beyond  the  scope  of  the  present  work.  For  the  con- 
venience of  the  reader  the  following  rules  and  recommendations 
of  the  Joint  Committee  formed  under  the  auspices  of  the 
Royal  Institute  of  British  Architects  are  printed  here  ;  the 
student  should  study  them  carefully. 


METHODS  OF  CALCULATION. 

1.  Loads. — In  designing  any  structure  there  must  be  taken  into 
account : — 

(a)  The  weight  of  the  structure. 

(6)  Any  other  permanent  load,  such  as  flooring,  plaster,  etc. 

(c)  The  accidental  load. 

(d)  In  some  cases  also  an  allowance  for  vibration  and  shock. 

Of  all  probable  distributions  of  the  load,  that  is  to  be  assumed  in 
calculation,  which  will  cause  the  greatest  straining  action. 

(i.)  The  weight  of  the  concrete  and  steel  structure  may  be  taken  at 
150  Ibs.  per  cubic  foot. 


Neutral   -  -  -X- 


FIG.  46. — Diagram  showing  Principal  Lines  of  Stress  in  Loaded  Beam. 


FIG.  47.— Shear  Diagram  for  Beam  under  gradually  increasing  Load. 

(ii.)  In  structures  subjected  to  very  varying  loads  and  more  or  less 
vibration  and  shock,  as,  for  instance,  the  floors  of  public  halls,  factories, 
or  workshops,  the  allowance  for  shock  may  be  taken  equal  to  half  the 
accidental  load.  In  structures  subjected  to  considerable  vibration  and 
shock,  such  as  floors  carrying  machinery,  the  roofs  of  vaults  under 
passageways  and  courtyards,  the  allowance  for  shock  may  be  taken 
equal  to  the  accidental  load. 

(iii.)  In  the  case  of  columns  or  piers  in  buildings,  which  support 


216  REINFORCED  CONCRETE 

three  or  more  floors,  the  load  at  different  levels  may  be  estimated  in 
this  way  :  — 

For  the  part  of  the  roof  or  top  floor  supported,  the  full  accidental 
load  assumed  for  the  floor  and  roof  is  to  be  taken. 

For  the  next  floor  below  the  top  floor  10  per  cent,  less  than  the 
accidental  load  assumed  for  that  floor. 

For  the  next  floor  20  per  cent,  less,  and  so  on  to  the  floor  at  which 
the  reduction  amounts  to  50  per  cent,  of  the  assumed  load  on  the  floor. 

For  all  lower  floors  the  accidental  load  on  the  columns  may  be  taken 
at  50  per  cent,  of  the  loads  assumed  in  calculating  those  floors. 

2.  Spans.  —  These  may  be  taken  as  follows  :   For  beams  the  distance 
from  centre  to  centre  of  bearings.     For  slabs  supported  at  the  ends, 
the  clear  span  +  the  thickness  of  slab.     For  slabs  continuous  over 
more  than  one  span,  the  distance  from  centre  to  centre  of  beams. 

3.  Bending  Moments.  —  In   the   most  ordinary  case  of   a   uniformly 
distributed  load  of  w  Ibs.  per  inch  run  of  span,  the  bending  moments 
will  be  as  follows  :  — 

(a)  Beam  or  Slab  simply  supported  at  the  ends.  —  Greatest  bending 
moment  at  centre  of  span  of  I  inches  is  equal  to  wl*  -=-  8  inch  Ibs. 

(b)  Beam  continuous  over  several  Spans,  or  Encastre  or  fixed  in  direction 
at  each  end.  —  The  greatest  bending  moments  are  at  the  end  of  the  span, 
and  the  beam  should  be  reinforced  at  its  upper  side  near  the  ends. 
If  continuity  can  be  perfectly  relied  on,  the  bending  moment  at  the 
centre  of  the  span  is  wl2  -*-  24,  and  that  over  the  supports  wl2  -=-  12.     If 
the  continuity  is  in  any  way  imperfect,  the  bending  moment  at  the 
centre  will,  in  general,  be  greater,  and  that  at  the  supports  less,  but  the 
case  is  a  very  indefinite  one.      It  appears  desirable  that  in   building 
construction  generally  the  centre  bending  moment  should  not  be  taken 
less  than  wl2  -=-12.     The  bending  moment  at  the  ends  depends  greatly 
on  the  fixedness  of  the  ends  in  level  and  direction.     When  continuity 
and  fixing  of  the  ends,  whether  perfect  or  imperfect,  is  allowed  for  in 
determining  the  bending  moment  near  the  middle  of  the  span,  the 
beam  or  slab  must  be  designed  and  reinforced  to  resist  the  corresponding 
bending  moments   at  the  ends.       When  the  load  is   not  uniformly 
distributed,  the  bending  moments  must  be  calculated  on  the  ordinary 
statical  principles. 

4.  Stresses.  —  The  internal  stresses  are  determined,  as  in  the  case  of 
a  homogeneous  beam,  on  these  approximate  assumptions  :  — 

(a)  The  coefficient  of  elasticity  in  compression  of  stone  or  gravel 
concrete,  not  weaker  than  1  :  2  :  4,  is  treated  as  constant,  and  taken 
at  one-fifteenth  of  the  coefficient  of  elasticity  of  steel. 

Ibs.  per  sq.  in. 

Coefficient  for  concrete  =  Ec  =    2,000,000 
„  steel         =  E8  =  30,000,000 


It  follows  that  at  any  given  distance  from  the  neutral  axis  the  stress 
per  square  inch  on  steel  will  be  fifteen  times  as  great  as  on  concrete. 

(b)  The  resistance  of  concrete  to  tension  is  neglected,  and  the  steel 
reinforcement  is  assumed  to  carry  all  the  tension. 

(c)  The  stress  on  the  steel  reinforcement  is  taken  as  uniform  on  a 
cross-section,  and  that  on  the  concrete  as  uniformly  varying. 

5.   Working  Stresses.  —  If  the  concrete  is  of  such  a  quality  that  its 
crushing  strength  is  2,400  to  3,000  Ibs.  per  square  inch  alter  twenty- 


METHODS  OF  CALCULATION  217 

eight  days,  and  the  steel  has  a  tenacity  of  not  less  than  60,000  Ibs.  per 
square  inch,  the  following  stresses  may  be  allowed  : — 

Ibs.  per  sq.  in. 

Concrete  in  compression  in  beams  subjected  to  bending  .  600 
Concrete  in  columns  under  simple  compression  .  .  500 
Concrete  in  shear  in  beams  .....  60 
Adhesion1  of  concrete  to  metal  .  .  .  .  .100 
Steel  in  tension 15,000  to  17,000 

When  the  proportions  of  the  concrete  differ  from  those  stated  above, 
the  stress  in  compression  allowed  in  beams  may  be  taken  at  one-fourth, 
and  that  in  columns  at  one-fifth  of  the  crushing  stress  of  cubes  of  the 
concrete  of  sufficient  size  at  twenty-eight  days  after  gauging.  If 
stronger  steel  is  used  than  that  stated  above,  the  allowable  tensile 
stress  may  be  taken  at  half  the  stress  at  the  yield  point  of  the  steel. 

When  the  foregoing  rules  are  familiar,  the  formulae  used  by 
specialists  in  concrete  construction2  may  then  be  studied  with 
advantage,  though  many  of  these  simplified  formulae  contain 
constants  and  other  somewhat  empirical  matter  which,  in  the 
hands  of  a  man  of  experience,  are  useful,  but,  if  used  by  a 
beginner,  may  lead  to  a  considerable  risk  of  error. 

Specialists  in  concrete  construction  have,  by  the  aid  of 
such  calculations,  devised  various  ingenious  arrangements  of 
the  steel  within  the  concrete  so  as  to  produce  structures  of 
enormous  strength  at  a  relatively  low  cost.  These  "  systems  " 
of  reinforcement  usually  bear  the  name  of  their  inventor,  or  give 
some  indication  of  the  shape  of  the  reinforcement  in  their 
titles.  Whilst  each  system  usually  has  some  advantage  over 
others,  no  definite  means  has  yet  been  formed  whereby  all 
points,  including  economy,  may  be  considered  and  balanced 
against  each  other.  Consequently,  it  is  very  difficult  for  an 
engineer  or  architect  who  has  not  specialised  in  reinforced 
concrete  to  determine  the  relative  merits  of  various  systems. 
To  attempt  to  decide  on  a  question  of  cost — leaving  the  respon- 
sibility of  selecting  the  proper  factors  of  safety  to  the  firms 

1  It  is  desirable  that  the  reinforcing  rods  should  be  so  designed  that  the  adhesion 
is  sufficient  to  resist  the  shear  between  the  metal  and  concrete.     Precautions  should 
in  every  case  be  taken  by  splitting  or  bending  the  rod  ends,  or  otherwise,  to  provide 
additional  security  against  the  sliding  of  the  rods  in  the  concrete.     [It  should, 
however,  be  noted  that  this  treatment  has  no  effect  on  the  greater  part  of  the 
lenath  of  smooth  bars,  but  only  near  the  ends  where  the  adhesion  is  least  liable  to 
be  destroyed.— A.  B.  S.] 

2  The  excellent  theoretical  analysis  in  the  handbook  supplied  by  the  manu- 
facturers of  the  Kahn  trussed  bar  is  well  worth  special  study,  as  it  is  a  particularly 
clear  enposition  of  the  "  Straight  Line  Formula,"  which  is  rapidly  becoming  of 
general  acceptance. 


218  REINFORCED  CONCRETE 

making  the  tenders — is,  of  all  ways,  the  most  unsatisfactory, 
and  is  sure,  at  some  time,  to  lead  to  trouble. 

The  generally  accepted  method  of  reinforcement  consists  in 
the  insertion  of  thin  horizontal  bars  or  rods  and  strips  of  steel 
in  just  those  places,  and  in  those  places  alone,  where  the 
resistance  of  the  concrete  requires  to  be  supplemented  in  order 
that  it  may  withstand  tensile  stresses.  These  bars  may  be 
bent  at  each  end  so  as  to  provide  an  increased  resistance  to 
the  shearing  force  supplied  by  the  load,  or  they  may  be  fitted 
with  "  shearing  members  "  in  the  shape  of  attached  bars,  loops, 
stirrups,  etc.  The  shape  of  these  attachments  and  of  the  bars 
themselves  is  the  subject  of  various  proprietory  rights,  and 
forms  the  chief  distinction  between  the  different  "  systems  " 
of  reinforced  concrete. 


FIG.  48. — Bending  Moment  of  Beam  under  gradually  increasing  Load. 

These  shear  members  are  necessarily  of  short  length,  since 
they  are  limited  by  the  depth  of  the  beam  itself,  which,  as  a 
rule,  does  not  exceed  one  or  two  feet. 

It  is,  therefore,  essential  that  these  ".shear  members" 
should  be  rigidly  attached  to  the  main  tensional  members, 
and  it  is  also  of  great  importance  that  a  good  mechanical 
bond  should  exist  between  the  steel  and  the  concrete  along 
the  shear  bars.  For  this  reason  the  R.I.B.A.  Committee 
Rules  (p.  215)  stipulate  that  :  "As  the  resistance  of  the  shear 
members  to  the  pull  depends  on  the  adhesion  and  the  anchorage 
at  the  ends,  it  is  desirable  to  use  bars  of  a  small  diameter,  and 
to  anchor  the  stirrups  at  both  their  ends."  If  a  smooth  bar 
is  bent  up  as  a  shear  bar  the  adhesion  of  the  concrete  to  the 
short  length  of  steel  available  is  insufficient  to  develop  the 
full  strength  of  the  metal  before  the  latter  will  pull  out  of  the 
concrete.  A  large  proportion  of  the  metal  in  the  shear  bars 
is  thus  wasted,  and  to  obtain  the  requisite  strength  far  more 
steel  has  to  be  provided  than  is  actually  necessary.  If,  how- 


SHEAR  MEMBERS  219 

ever,  indented  bars  (p.  235)  are  used,  the  full  strength  of  the 
steel  is  brought  into  action.  Some  of  the  main  tensional 
members  can  themselves  be  bent  up  towards  the  ends  of  the 
beam  into  the  correct  position  for  taking  the  shearing  stresses. 
These  "  shear  members  "  are  not  merely  connected  rigidly  to 
the  main  bars,  but  are  actual  portions  of  the  same  bars,  the 
anchorage  extending  throughout  their  length  (Fig.  49). 

The  L.C.C.  rules  refer  to  this  property  of  indented  bars 
when  they  demand  that  all  shear  bars  shall  have  a  "  mechanical 
anchorage  at  both  ends,  or  they  shall  have  a  mechanical  bond 
with  the  concrete  throughout  their  length." 

In  ordinary  commercial  round  or  square  bars,  small  sections 
are  generally  favoured,  as  they  give  a  larger  proportionate 


FIG.  49.  — Indented  Bais  bent  to  form  bliear  Members. 

surface  for  adhesion,  and  are  more  easily  manipulated.  Hoops, 
bands,  and  flats  of  small  section  are  used,  but  rounds  and 
squares  are  the  sections  generally  employed  in  reinforced 
concrete  work.  Other  sections,  more  particularly  T-bars,  are 
specially  fitted  for  reinforcing  arches  and  tall  chimneys,  while 
the  old  type  of  steel  and  concrete  floor,  of  which  there  is  a 
considerable  amount  still  constructed,  has  small  I  joists. 

Some  patent  bars  for  reinforcement  are  moulded  in  various 
shapes  and  sizes  with  the  idea  of  providing  the  mechanical 
bond,  though  numerous  authoritative  investigations  have 
shown  these  to  be  unnecessary  in  many  cases,  owing  to  the 
natural  adhesion  of  the  concrete  to  the  steel. 

The   arrangement   of   these   bars   as   well   as   their   shapes, 


220  REINFORCED  CONCRETE 

constitute  the  foundations  of  the  various  "  systems  "  now  in 
use.  Each  fundamentally  different  structure  requires  a 
different  arrangement  ;  thus,  independent  spread  foundations 
are  now  reinforced  with  bars,  although  the  older  form  of  steel 
joists,  crossing  each  other,  is  still  much  used.  Where  raft 
foundations  are  used,  a  reinforced  concrete  raft,  thoroughly 
framed  together  with -beams  forming  ribs,  can  be  constructed 
very  much  thinner  than  one  which  is  not  reinforced,  and  gives 
security  against  unequal  loading  and  unequal  support  from 
the  subsoil,  and  against  shocks  and  vibrations  of  great  magni- 
tude, so  that  such  rafts  are  particularly  applicable  in  districts 
subject  to  earthquakes. 

In  reinforced  concrete  work,  most  buildings  are  carried  out 
on  the  frame  principle,  the  loads  being  carried  from  the  beams 
on  to  columns,  and  the  walls  are  mere  partitions  between  them. 

In  ordinary  cases,  there  is  very  little  lateral  pressure  on 
walls,  and  an  ordinary  square  mesh- work  embedded  is  sufficient. 

Many  retaining  walls  have  been  built  in  reinforced  concrete. 
Steel  reduces  the  thickness  and  the  amount  of  material  to  be 
excavated,  because  buttresses,  with  thin  intermediate  slabs 
and  a  projecting  foot,  can  be  designed  to  be  thoroughly  stable 
against  overturning  and  shear.  There  are  no  special  systems 
with  regard  to  reinforcing  retaining  walls  that  call  for  notice. 

A  cantilever  retaining  wall  is  a  striking  example  of  the 
enormous  economy  of  reinforced  concrete  as  compared  with 
mass  concrete,  brickwork  or  masonry,  the  precise  type  of  wall 
being  selected  according  to  the  requirements  of  the  case.  For 
instance,  in  the  retaining  walls  of  the  Royal  Insurance  Offices, 
in  Piccadilly,  the  thrust  of  the  earth  is  taken  by  a  vertical 
wall,  and  a  horizontal  slab  occurs  on  the  opposite  face  to  the 
earth  pressure.  The  horizontal  beams  at  the  top  do  not 
afford  any  horizontal  support  to  the  wall.  This  wall, 
constructed  in  1908  of  concrete  reinforced  with  indented  bars, 
was  the  first  of  its  type  to  be  built,  but  several  have  since 
been  constructed.  Counterforts  could,  of  course,  be  used,  but 
the  cantilever  type  adopted  in  this  instance  affords  a  clear 
run  immediately  behind  the  wall,  and  affects  an  enormous 
saving  in  space  as  compared  with  a  counterfort  wall,  or  still 
more  in  comparison  with  a  mass  concrete  or  masonry  wall, 


RETAINING  WALLS 


221 


which  would  have  been  approximately  ten  feet  wide  at  the 
base  instead  of  two  feet  six  inches  as  built.  Another  excellent 
'example  of  the  use  of  reinforced  concrete  (with  indented  bars) 
in  place  of  mass  concrete  is  the  retaining  wall  for  Self  ridges, 
which  is  probably  the  deepest  in  London,  being  sixty  feet  deep 
and  twelve  feet  thick  at  the  base.  The  load  on  the  piers 
which  support  the  building  is  carried  by  a  heavy  reinforced 
concrete  slab.  This  wall  provides  three  basement  floors  below 
street  level ;  it  is  of  cantilever  type,  similar  to  that  in  the 
Royal  Insurance  Offices  just  described. 

Columns,  piers,  posts,  or  stanchions  are  constructed  of 
reinforced  concrete,  the  reinforcement  being  used  to  reduce 
the  section  of  the  concrete,  to  bind  it  together,  and  to  prevent 


1  i  I  I  i 


f' 

/ 

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1  f 

! 

\ 

1  I 

\  \ 

\   ' 

\ 

\ 

. 

\ 

\ 

\ 

/ 

-7 
/ 
1 
I 
\ 

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i 
/ 

1 

1 
\ 
\ 
\ 

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i 
i 

/ 

FIG.  50.— No  Lateral 
Ties. 


FIG.  51.— One 
Lateral  Tie. 


> 
i 

i 
i 

> 

/ 

i 

IG.  52.  —  Sever* 
Lateral  Ties. 

bulging.  For  these  articles  the  concrete  usually  consists  of 
a  1:2:4  mixture.  A  column  provided  with  longitudinal 
reinforcement  only  would  bulge  or  swell  when  overloaded 
(Fig.  50).  Hence,  these  longitudinal  bars  must  be  tied 
together  by  a  series  of  transverse  bonds.  Four  types  of  these 
are  used  :  (a)  circular  hoops  or  rings  placed  at  a  convenient 
distance  apart,  (6)  rectilinear  ties  made  of  wire,  similarly 
placed  (Fig.  51),  (c)  a  spiral  wire  (Fig.  52),  and  (d)  a  network 
surrounding  the  vertical  bars. 

The  use  of  these  ties  was  originated  by  F.  Hennebique,  and 
has  been  applied  extensively  with  satisfactory  results.  For 
difficult  cases  it  is  well  to  bear  in  mind  that  there  is  evidence 
to  show  that  the  greatest  strength  is  obtained  with  longitudinal 


222 


REINFORCED  CONCRETE 


FIG.  54. 


bars  surrounded  by  spiral  reinforcement,  the  distance  between 
the  coils  being  small  enough  to  resist  the  lateral  or  radial 
expansion  of  the  concrete  (this  is  the  basis  of  the  Considere 
System).  Jointed  circular  hoops  and  horizontal  wire  ties  are 
slightly  inferior,  though  for  ordinary  cases  they  are  of  more 
than  ample  strength. 

Tests  of  concrete  columns  with  short  lengths  of  longitudinal 
reinforcement  in  combination  with  continuous  transverse 
reinforcement  show  that  this  is  the  weakest  form.  The 
reinforcements  consist  of  longitudinal  bars  and  horizontal 
ligatures,  the  latter  being  formed 
either  of  links  or  spirally-wound 
rods  of  smaller  section  than  the 
longitudinal  bars. 

The  use  of  hoops,  network,  or 
of  a  spiral  wire  without  longitudinal 
bars  has  proved  unsatisfactory 
when  tested,  the  metal  in  this  form 
of  reinforcement  being  able  to 
develop  only  a  small  fraction  (about  2  per 
cent.)  of  its  full  strength,  and  is  therefore 
wastefully  applied.  The  use  of  longitudinal 
bars  of  ample  size  running  the  whole  length 
of  the  column  and  surrounded  by  suitable  ties 
affords  an  ample  reserve  of  strength. 

The  use  of  longitudinal  bars  fitted  with  wings  or  inclined 
members,  but  without  ties,  is  not  satisfactory  for  column 
construction.  If  ties  are  used  the  inclined  members  are 
unnecessary,  and  without  the  ties  the  column  is  weak. 

The  objection  to  a  mesh  or  network  for  reinforcing  columns 
is  the  difficulty  of  making  a  secure  joint  along  the  edges  of  the 
material  and  of  tamping  the  concrete  into  the  meshes  of  the 
network.  This  difficulty  is  often  exaggerated  by  the  represen- 
tatives of  other  systems  of  reinforcement,  though  it  does 
undoubtedly  exist.  Attempts  to  overcome  it  by  using  a  more 
fluid  concrete  should  be  resisted  strenuously  (see  p.  234). 

To  overcome  the  inherent  weaknesses  of  the  foregoing 
systems — especially  the  strains  created  in  the  metal  by  longi- 
tudinal compression  on  account  of  the  setting  of  the  concrete, 


FIG.  53.— 

Hennebique 

Column. 


COLUMNS,  PIERS  AND  STANCHIONS  223 

and  the  consequent  tensional  stress  in  the  concrete — the 
reinforced  metal  type  of  column  (Fig.  55)  has  been  devised 
(British  Patent  27529,  1910). 

It  consists  of  a  single  axially-disposed  steel  cylinder,  either 
hollow  or  solid,  or  a  star-shaped  rolled  steel  section,  which 
furnishes  the  longitudinal  reinforcement,  and  serves  as  anchor- 
age for  four  steel  spirals  which  are  disposed  eccentrically 
around  it.  These  spirals  embrace  the  core  and  on  their  inner 
side  bear  hard  against  it ;  they  intermesh  with  one  another, 


FIG.  55.- — Core  with  Anchored  Spirals.     End  View. 
(Courtesy  of  Reinforced  Metal,  Ltd.) 

giving  lateral  reinforcement  at  every  point  throughout  the 
column  and  constitute  shear-resisting  members  of  a  most 
effective  character.  The  area  of  steel  when  a  solid  core  is  used 
is  12-56  per  cent.,  that  when  a  hollow  core  is  used  2'4  per 
cent,  of  the  area  of  the  column  within  the  reinforcement,  but 
the  percentage  of  steel  reinforcing  each  compartment  relatively 
to  the  area  of  its  contents  is  about  five  times  as  great  as  would 
be  the  case  with  the  ordinary  spirally-hooped  column  were  the 


224 


REINFORCED  CONCRETE 


same  total  weight  of  spiralling  employed  in  both.  The  peculiar 
characteristic  of  this  arrangement  is  that,  as  shown  by 
Professor  A.  Gray's  test,  its  modulus  of  elasticity  under 

progressive  loading  increases  with 
increase  of  compressive  stress — a 
property  possessed  by  no  other 
combination  of  materials  yet  in- 
vestigated. 

The  compressive  strength  of  a  re- 
inforced metal  column  is  stated  by 
the  owners  of  the  patents  to  be 
twice  that  of  spirally-hooped  con- 
crete column  and  three  times  that 
of  a  steel  stanchion  encased  in 
concrete  in  which  the  same  weight 
of  longitudinal  steel  and  concrete  is 
used. 

It  is  claimed  that,  as  the  materials 
do  not  cost  more  to  buy  nor  the 
column  more  to  make  than  others, 
the  cost  of  the  Reinforced  Metal 
Column  per  ton  of  load  capacity  per 
about  one-half  that  of  reinforced 


FIG.  56.— Column  Base 
(Hennebi^ue). 


foot   of    length   is   only 
concrete  columns. 

Column  bases  may  advantageously  be  reinforced  in  two 
horizontal  directions, 
each  at  right  angles  to 
the  other  (Figs.  56 
and  57)  so  as  to  take 
up  all  tensional  strain 
which  would  other- 
wise result  in  a 
spreading  of  the  base. 


/O" 

w\ 

\ 

8 

: 

Stirrups 

::H*:  ••••* 

': 

]^-»^,-^!iim 

Beams,  girders  and 
stanchions  are  usually 


FIG.  57. — Column  Base  ((Joignet). 


made  of  a  1:2:4  mixture  reinforced  with  one-inch  rods  of 
mild  steel,  with  a  network  reinforcement  or  with  patent  bars. 
The  reinforcement  of  beams  —  one  of  the  most  important 
branches  of  concrete  structural  work  —  is  usually  placed  near 


BEAMS  225 

to  the  lower  side  of  the  beam,  except  above  the  supports 
where  it  is  bent  upwards  so  as  to  lie  near  the  top  of  the 
beam. 

The  reason  for  this  is  that  the  parts  of  a  beam  which  lie  in 
the  centre  of  the  span  between  the  columns  have  the  lower 
part  in  tension  and  the  upper  part  in  compression.  For  a 
short  distance  over  each  column,  however,  the  nature  of  the 
stresses  is  reversed,  and  the  upper  part  of  the  beam  is  in  tension 
and  the  lower  part  in  compression. 

A  further  provision  of  steel  is  necessary  by  way  of  web 
reinforcement  to  resist  shear.  The  function  of  web  reinforce- 
ment is  to  connect  together  the  compression  and  tension  areas 
in  a  rigid  manner  in  precisely  the  same  way  as^the  bars  of  a 
lattice  girder  or  the  members  of  a  truss  girder. 


FIG.  58. 

The  shear  which  these  members  resist  is  the  greatest  at  the 
ends  of  a  uniformly  loaded  beam  and  diminishes  towards  the 
centre.  It  is  therefore  necessary  that  shear  reinforcement 
should  be  heavier  at  the  ends  than  at  the  centre,  or  if  the  shear 
bars  are  of  the  same  section  throughout,  they  should  be 
placed  at  gradually  decreasing  distances  from  the  centre  to  the 
ends  of  the  beam.  The  three  principal  points  of  reinforcement 
in  beams  are,  therefore  : — 

(a)  The  main  tension  bars  at  the  bottom  of  the  beam. 

(b)  The  tension  bars  at  the  top  of  the  beam  at  points  of 
contraflexure  over  columns. 

(c)  The  web  or  shear  reinforcement  connecting  the  tension 
and  compression  areas  together. 

Where  the  beam  is  large  and  T-shaped,  supplementary 
reinforcing  bars  are  also  placed  in  the  head  of  the  T  and  at 
right  angles  to  the  others.  The  same  arrangement  is  used  in 
reinforced  floors  carried  on  beams  (Fig.  58), 

c.  Q 


226 


REINFORCED  CONCRETE 


As  the  actual  amount  of  diagonal  tension  at  any  point  in 
a  beam  cannot  be  determined  with  accuracy,  it  is  customary 
to  calculate  the  vertical  shearing  stress,  and  to  use  that  as  a 
convenient  measure  of  the  diagonal  tensile  stress.  Hence  the 
survival  of  the  terms  "  shearing  failure  "  and  "  shear  reinforce- 
ment," though  in  reality  the  form  of  failure  in  beam  tests, 
commonly  described  as  a  "  shearing  failure,"  is  almost 
invariably  a  "  diagonal  tension  failure." 

The  investigations  of  M.  Feret,  Professor  Spofforth,  Professor 
Talbot  and  others,  have  shown  that  the  shearing  strength  of 
concrete  may  be  taken  as  two-thirds  of  its  compressive  strength, 
so  that  there  is  little  risk  of  failure  by  vertical  shear,  even  in 
a  plain  concrete  beam,  but  failure  will  occur  by  diagonal 


Section  on  A-b.  Section  on  C'D  Section  en  E-F. 

FIG.  59. — Reinforced  Beam  (Hennebique  System). 

tension  when  the  vertical  shearing  stress  reaches  a  very 
moderate  intensity,  which  Professor  Talbot  has  found  to  vary 
from  70  Ibs.  to  140  Ibs.  per  square  inch,  according  to  the  quality 
of  the  concrete  used.  Thus,  although  the  shearing  stress  may 
not  in  itself  be  sufficient  to  affect  the  concrete,  a  comparatively 
small  stress  of  this  kind  generally  indicates  the  presence  of  a 
dangerous  amount  of  diagonal  tension.  Hence,  it  is  evident 
that  the  shearing  stress  of  concrete — regarded  as  the  measure 
of  diagonal  tension — must  be  kept  within  limits  which  are 
so  small  as  to  be  practically  negligible.  Thus,  the  practice  of 
making  no  allowance  for  the  shearing  resistance  of  concrete  is 
fully  justified,  and  the  recommendation,  made  by  some  other 
engineers,  that  concrete  in  shear  may  be  subject  to  a  stress  of 
50  Ibs.  or  60  Ibs.  per  square  inch,  would  leave  too  small  a 
margin  of  safety  against  failure  by  diagonal  tension. 


BEAMS 


227 


Whilst  failure  by  diagonal  tension  may  be  expected  to  occur 
in  a  beam  having  no  web  reinforcement  if  the  shearing  stress 
intensity  were  to  attain  about  100  Ibs.  per  square  inch,  recent 
well-authenticated  tests  have  shown  that  beams  when  properly 
reinforced  are  capable  of  withstanding  shearing  stresses  of 
more  than  600  Ibs.  per  square  inch 
without  giving  the  slightest  indication 
of  failure  by  diagonal  tension. 

One  of  the  earliest,  and  still  one 
of  the  most  prominent,  systems  of 
reinforcing  beams  is  that  invented  in 
1892  by  F.  Hennebique  (Fig.  59),  and 
used  in  the  General  Post  Office,  London, 
and  numerous  other  large  buildings. 
Round  bars  are  used,  arranged  as 
shown.  The  bars  are  hung  from 
strips  bent  in  the  form  of  stirrups,  the  latter  taking  the 
shear,  and  the  inclined  bars  the  diagonal  tension.  These 
stirrups  are  placed  uniformly,  except  at  the  ends  of  the  span 
where  the  shear  stress  is  greater ;  there  they  are  placed  closer 
together  (Figs.  60  and  61). 

The  bent  up  ends  of  the  bars  lie  fairly  across  the  lines  of 
rupture  near  the  supports  of  the  beam,  and  afford  in  them- 
selves very  secure  anchorage.  The  vertical  stirrups  are  formed 


FIG.  60.— Hennebique 
Stirrup  and  Bar. 


FIG.  61. — Bars  and  Stirrups  (Hennebique  System). 

so  that  they  can  be  spaced  at  proper  intervals  along  the  beam 
to  provide  for  variations  of  stress  from  point  to  point.  They 
cross  numerous  lines  of  maximum  tension  (Fig.  68),  are,  there- 
fore, of  great  efficiency,  and  being  vertical  they  facilitate  the 
operation  of  ramming  the  concrete  without  causing  the  risk  of 
displacement.  Being  made  with  a  simple  spring  clip  at  the 
lower  end,  the  stirrups  are  automatically  held  in  position  on 
the  main  bars,  thereby  obviating  the  necessity  for  temporary 
wedges  (Fig.  63)  or  ties.  Finally,  the  upper  ends  of  each 

Q2 


228 


REINFORCED  CONCRETE 


stirrup  are  bent  over  at  right  angles  so  as  to  ensure  perfect 
anchorage  in  the  concrete,  and  to  make  each  stirrup  an  efficient 
connection  between  the  tension  and  compression  areas  of  the 
beam  in  which  they  are  embedded. 

In  discussing  the  relative  efficiency  of  inclined  and  vertical 
stirrups,  Professor  Turneaure  has  pointed  out  that  while  an 
inclined  stirrup,  or  the  bent  end  of  a  horizontal  bar,  is  in  a 
position  to  take  stress  immediately,  a  vertical  stirrup  is  more 
effective  in  resisting  vertical  distortion  of  the  concrete.  In 
his  opinion,  the  stirrups  should  be  looped  around  the  horizontal 
bars  so  as  to  be  firmly  anchored  at  their  lower  end  where  the 
stress  is  a  maximum,  but  that  attachment  to  the  bars  is  not 
necessary,  as  the  object  of  the  stirrup  is  to  prevent  vertical 
or  nearly  vertical  distortion. 

The  Hennebique  vertical  stirrups  also  form  an  effective 
web-connection  between  the  tension  and  compression  portions 


FIG.  62. — Reinforcement  for  Beam  with  Compression  Bar  and 
Double  Stirrups  (Hennebique). 

in  reinforced  beams.  It  has  been  shown  (Fig.  68)  that  the  lines 
of  maximum  tension  assume  diagonal  directions  towards  the 
ends  of  a  beam,  and  these  can  be  resolved  into  two  components, 
one  vertical  and  the  other  horizontal,  the  former  being  taken 
entirely  by  the  stirrups  and  the  latter  by  the  horizontal  bars. 
This  fact  is  made  use  of  in  advocating  the  adoption  of  vertical 
stirrups,  as  in  the  Hennebique  system,  in  preference  to  inclined 
ones. 

The  mechanical  bond  in  the  Hennebique  system  is  secured 
by  flattening  and  opening  out  the  ends  of  all  bars  so  as  to  form 
a  secure  anchorage,  and  even  in  the  most  simple  beams  at 
least  half  the  bars  are  bent  up  towards  the  supports,  thus 
giving  further  security. 

Where  unusually  heavy  loads  have  to  be  carried  by  beams, 
whose  dimensions  must  be  kept  within  comparatively  small 
limits  in  order  to  comply  with  structural  or  architectural 


HENNEBIQUE  SYSTEM  229 

requirements,  and  in  some  structures  where  the  spans  of 
continuous  beams  are  liable  to  variable  and  unequal  loadings, 
such  as  the  main  beams  in  bridges,  viaducts,  wharves,  piers  and 
jetties  subject  to  heavy  rolling  loads,  as  of  railway  rolling- 
stock  or  other  vehicles  carrying  considerable  weights,  it  is 
desirable  to  use  compressional  reinforcement.  In  the  Henne- 
bique  system  the  beams  are  provided  with  stirrups,  as  before 
described,  for  withstanding  tension  on  diagonal  planes,  and 
in  addition  with  a  series  of  inverted  stirrups  passing  over  the 
upper  bars  and  anchored  in  the  lower  part  of  the  beam.  Both 
the  compression  and  tension  bars  are  carried  across  the  supports 
or  through  columns,  and  so  perfect  connection  is  provided 
between  adjoining  spans. 

The  complete  arrangement,  illustrated  in  Fig.  62,  shows  that 
the  systenTof  main  bars  and  double  stirrups  ensures  ample 


FIG.  63. — Keedon  Bar  for  Beams. 

resistance  to  horizontal  tension  and  compression,  and  to  tension 
and  compression  in  diagonal  directions,  while  at  the  same  time 
it  constitutes,  after  incorporation  in  the  surrounding  concrete, 
a  truss  of  great  rigidity  and  extraordinary  capacity  for  with- 
standing deflection. 

Although  one  of  the  earliest  forms  of  reinforcement,  the 
general  arrangement  of  the  bars,  stirrups  and  ties  originated 
by  F.  Hennebique  still  form  the  basis  of  the  most  important 
"  systems  "  now  in  use.  The  variations  made  by  other 
engineers  and  patentees  occur  chiefly  in  the  shape  and  mode  of 
attachment  of  the  shear  members,  in  the  use  of  bars^  of  special 
surface  intended  to  secure  greater  adhesion  between  the 
concrete  and  steel,  and  in  the  elimination  of  certain  members 
or  the  reduction  in  size  of  others,  with  a  view  to  reducing  the 


230  REINFORCED  CONCRETE 

amount  of  steel  used.  Some  of  these  "  improvements  "  are 
of  great  value,  e.g.,  the  rigid  attachment  of  the  shear  members, 
and  the  indentation  of  the  surface  of  the  main  bars — but  they 
are  usually  accompanied  by  other  disadvantages,  so  that  the 
engineer  or  architect  must  study  the  whole  matter  with  full 
regard  to  local  requirements  before  selecting  any  "  system  " 
or  arrangement  to  suit  a  particular  case. 

If  workmen  could  be  relied  upon  to  follow  instructions 
implicitly,  and  if  the  time  occupied  were  of  secondary  impor- 
tance, there  can  be  no  question  that  the  arrangement  of  the 
shear  members  to  suit  each  case  as  it  may  arise  would  be 
best,  and  the  Hennebique  system  would  then  be  the  best 
basis  on  which  to  work.  As  the  risk  of  inserting  the  shear 
members  in  the  wrong  places,  and  of  displacing  them  during 
tamping,  to  say  nothing  of  other  carelessness,  must  be  reckoned 
with  in  actual  practice,  the  use  of  one  of  the  following  systems 


FIG.  64. — Coignet  Reinforcement  for  Beams  (latest  type). 

is  often  convenient  and  may,  on  occasion,  prove  even  more 
satisfactory.  This  is  not  so  much  due  to  any  defect  in  the 
Hennebique  system,  but  to  the  workmen  using  it,  and  the 
aim  of  the  engineer  and  architect  using  concrete  should  always 
be  to  secure  the  best  results  whilst  leaving  as  little  as  possible 
to  the  intelligence  or  integrity  of  the  workmen  employed. 

For  all  ordinary  structures,  the  designs  of  reinforcement 
suggested  by  different  firms  of  "  concrete  specialists  "  approach 
so  nearly  to  a  common  standard  that  there  is  little  to  choose 
between  them.  Each  of  these  firms  can  offer  an  abundance 
of  evidence  of  such  good  work  that  on  this  alone  no  decisive 
choice  can  be  made. 

For  special  purposes,  on  the  contrary,  each  of  the  leading 
designs  has  some  characteristics  of  value  not  possessed  by  the 
others,  and  the  architect  or  engineer  must,  therefore,  study 
these  variations  in  detail,  and  choose  the  system  which  is 
best  adapted  to  the  special  needs  of  the  case. 


COIGNET  SYSTEM 


231 


In  the  system  invented  by  E.  Coignet  (Figs.  64  and  65),  round 
bars  are  also  used  together  with  transverse  rods  of  smaller 
diameter  on  the  upper  or  compression  side.  This  enables  the 
main  beams  to  be  made  first,  then  hoisted  into  position  and  the 
floor  slabs  fitted  afterwards.  The  stirrups  in  this  system  consist 
of  round  bars  three-sixteenths  to  one- quarter  inch  in  diameter, 
the  ends  of  which  are  twisted  to  form  a  loop.  These  loops  are 
placed  over  both  top  and  bottom  bars,  and  are  tied  in  place 
with  wire. 

The  Kahn  Trussed  Bar  (Fig.  66)  is  of  special  shape,  the  section 
of  the  main  portion  being  that  of  a  diamond.  This  bar  is 
provided  at  frequent  intervals  with  supplementary  bars  bent 


FIG.  65.— Section  of  Coignet  Floor  (old  type). 


at  an  angle  of  45  degrees  so  as  to  lie  along  the  principal  lines 
of  stress  (Fig.  67),  and  at  right  angles  to  the  main  lines  of 
rupture. 

The  objection  to  this  arrangement  is  that  the  lines  of  maxi- 
mum tension  only  assume  the  angle  of  45  degrees  at  the  neutral 
axis  and  thereby  reduce  the  value  of  the  bent  bars,  and  in 
practice  they  are  almost  certain  to  be  bent  during  the  ramming 
or  tamping  of  the  concrete  unless  an  undesirably  fluid  mass 
is  used. 

When  the  supplementary  bars  or  stirrups  are  inclined  and 
rigidly  connected  to  the  bar,  thus  delivering  their  strain  into 
it,  the  tensile  stress  then  existing  in  the  horizontal  reinforce- 
ment is  not  only  that  caused  by  the  adhesion  of  the  concrete 


232  REINFORCED  CONCRETE 

to  it,  but  also  the  summation  of  the  horizontal  components 
of  the  strain  in  each  of  the  diagonals.  We  then  notice  that 
the  principles  of  truss  action  begin  to  appear  (see  Fig.  68). 
By  embedding  the  bars  above  described  in  concrete,  a  com- 
posite truss  is  formed  in  which  the  tension  members  are  steel, 
and  the  missing  compression  members  are  furnished  by  the 
concrete. 


Note 

rigid 
connection 


FIG.  66.— Kahn  Trussed  Bar. 

The  rigid  connection  of  the  bent  bars  with  the  large  hori- 
zontal bar  is  claimed  as  a  special  advantage  by  the  makers, 
who  rightly  state  that  the  transfer  of  the  stress  from  the  shear 
members  to  the  main  member  can  only  be  accomplished  by 
some  definite  connection  between  them  which  can  only  be 
obtained  by  a  rigid  attachment. 

It  is  claimed  by  some  engineers  that  the  concrete  surrounding 


KAHN  BARS 


233 


Gcction  of  Bor 


the  bars  will  prevent  the  slipping  of  the  loose  stirrups  or  tied- 
on  bars,  but  actual  tests  have  proved  that  slight  slipping  does 
sometimes  occur.  The  objection  to  all  bars  in  which  the 
auxiliary  reinforcement  is  placed  at  fixed  points  is  that  the 
position  of  every  bar,  rod,  or  strip  of  steel,  if  of  uniform 
section,  ought  to  be  settled 
by  the  designers  of  such 
bars  conformably  with  the 
stress  diagram  for  each  e  ^FIG  67^ 

structural  member,  or,   if 

the  auxiliary  or  web  reinforcement  is  in  the  form  of  bars 
placed  at  equal  distances  apart,  then  the  cross-sectional 
area  of  the  bars  should  be  progressively  varied  from  point  to 
point.  Neither  of  these  methods  of  variation  is,  however, 
possible  in  the  case  of  patent  bars  where  the  web  reinforcement 
is  formed  by  shearing  and  bending  up  part  of  a  projecting 
flange  rolled  with  the  main  bar. 

A  practical  disadvantage  of  bars  with  web  members  attached 
at  the  sides  is  the  excessive  width  of  the  bars  in  proportion 


'.>r>ci  of  ^rinci.oa/ compresses  Sfrt  33 


i 1 

Lrrics   of  prinr/uol 


.  68.  —  Showing  position  of  Shear  Members  in  Kahn  Bar 
and  Lines  of  Stress  in  Beam. 


to  their  tensile  resistance.  In  consequence,  bars  of  this  type 
cannot  be  used  with  economy  in  beams  having  to  carry  heavy 
loads,  as  the  width  of  the  beams  must  be  very  great  to  enable 
the  requisite  number  of  bars  to  be  applied,  the  excessive  width 
naturally  representing  waste  of  material  and  labour.  More- 
over, the  width  of  the  bars  is  apt  to  constitute  an  obstacle 
to  the  flow  of  concrete,  making  it  difficult,  if  not  impossible, 
to  fill  every  part  of  the  moulds,  and  to  obviate  the  presence  of 


234  REINFORCED  CONCRETE 

voids  between  and  under  the  reinforcement.  The  use  of  a 
very  fluid  concrete  only  introduces  more  serious  difficulties, 
as  it  fills  the  moulds  with  concrete  of  irregular  composition, 
because  the  aggregate  and  heavier  particles  of  sand  must 
necessarily  settle  to  the  bottom. 

The  most  rational  method  in  the  design  of  reinforcement, 
whether  in  beams,  columns,  piles,  or  other  structural  details, 
is  to  employ  forms  of  steel  which  permit  the  engineer  to  vary 
the  spacing  of  the  main  and  auxiliary  bars  at  pleasure,  and 
to  determine  by  calculation  the  number  and  sectional  area  of 
the  stirrups  as  demanded  by  the  intensity  and  distribution  of 
the  stresses  in  every  part  of  the  construction. 

That  is  the  method  adopted  in  the  Hennebique  System  ;   it 


PIG.  69.— Moss  Bar. 

is  generally  approved  by  some  of  the  most  eminent  authorities 
and  confirmed  by  the  many  perfectly  satisfactory  structures 
in  which  it  has  been  used,  which  occur  in  every  part  of  the  world. 
Yet  there  are  many  engineers  and  architects  who  feel  that  they 
cannot  trust  the  workmen  engaged  in  placing  the  concrete  to 
take  sufficient  care  in  putting  the  stirrups  in  the  corrrct 
positions  and  in  ensuring  their  non-disturbance  during  tamping. 
Such  engineers — and  they  are  very  numerous — are  naturally 
willing  to  forego  some  of  the  minor  advantages  of  the  loose 
stirrups  in  order  to  secure  the  shear  members  from  slipping 
out  of  place  and  so  rendering  possible  a  serious  collapse  of  the 
whole  structure.  Consequently,  they  prefer  fixed  bars  of  the 
Kahn,  Moss  or  similar  type. 

When  used  for  columns,  Kahn  bars  are  placed  vertically, 
one  near  each  corner  of  a  square  a  little  smaller  than  the 


MOSS  SYSTEM 


235 


average  area  of  the  column,  with  the  trusses  or  stirrups  pro- 
jecting towards  the  interior  of  the  column  (p.  222). 

In  the  Moss  system  (Figs.  69  and  70)  the  bar  is  of  I  section 
with  a  large  bottom  flange  ;  inverted  stirrups  are  fastened  along 
the  bar,  and  at  an  angle  to  it  so  as  to  form  trusses  with  some 
resemblance  to  the  Kahn  bars.  The  arrangement  of  the 
stirrups  depends  upon  the  loading  of  the  bar. 

The  Indented  Bar  system  uses  bars  of  square  or  round  section 
with  projections  or  indents  on  each  side  (Fig.  49  and  71),  these 
indents  being  sufficiently  deep  to  prevent  the  concrete  slipping 
along  the  bar.  These  bars  are  made  of  steel  with  a  somewhat 
higher  proportion  of  carbon  than  is  usual  in  reinforcement,  as 
such  steel  can  be  stressed  more  severely.  The  ends  of  .the  bars 
are  bent  where  required  to  take  additional  shear  stresses. 


None  THE  POSITIVE   BOND 
WITH  THE  RIGID  CONNECTION 


FIG.  70. — Isometric  View  of  Moss's  Patent  Girder  Reinforcement. 

In  addition  to  the  advantages  mentioned  on  p.  219,  the 
makers  of  the  indented  bars  claim  that  with  their  bars  "  there 
is  so  close  a  bond  between  the  concrete  and  the  steel  that  it 
prevents  undue  extension  of  the  concrete  towards  the  beam 
ends,  and  the  concrete  is  thus  bound  together  where  it  is 
exposed  to  the  greatest  shearing  stress  and  diagonal  tension, 
so  that  it  is  able  to  resist  shear  of  itself  to  a  far  greater 
extent  than  is  the  case  when  diagonal  tension  cracks  can 
take  place  in  the  concrete  and  thus  reduce  its  resistance. 
There  is  no  doubt  that  the  concrete  itself  plays  a  very 
important  part  in  resisting  shear  when  it  is  absolutely  prevented 
from  cracking  by  the  use  of  a  mechanical  bond  bar." 

A  further  advantage  claimed  in  favour  of  the  use  of  indented 
bars  is  that,  with  them,  "  tests  and  actual  practice  conclusively 


236 


REINFORCED  CONCRETE 


prove  that  no  cracks  of  sufficient  size  to  admit  moisture  to  the 
steel  can  occur  in  the  concrete  until  the  yield  point  of  the  steel 
has  been  reached.  At  this  point,  however,  the  whole  member 
will  be  practically  disintegrated  owing  to  the  complete  inability 
of  the  concrete  to  stretch  to  the  same  extent  as  the  steel. 
In  other  words,  the  yield  point  of  steel  reinforcement  is  the 
critical  point  at  which  the  structure  will  fail,  and  it  is  utterly 
fallacious  to  consider  the  ultimate  or  breaking  strength  of  the 
steel  as  the  critical  point." 

The  objection  is  sometimes  raised  that  the  ridges  and  sharp 
angles  on  indented  bars  injure  the  concrete  when  the  structural 

members  are  in  a 
state  of  strain.  The 
effect  is,  however,  too 
small  to  be  appre- 
ciated in  practical 
construction,  and  no 
failures  have  yet 
occurred  through  it. 

Spiral  Bars  have 
spiral  grooves  run- 
ning along  their  outer 
surface  and  have  the 
advantage  over 

smooth,  round  bars  of  rendering  crooking  or  fishtailing  the 
ends  of  tension  bars  unnecessary.  The  spiral  grooves  secure 
a  strong  mechanical  bond  and  enable  the  bars  to  be  used  for 
working  stresses  up  to  30,000  Ibs.  per  square  inch  or  50  per 
cent,  more  than  is  the  case  with  smooth  bars ;  consequently, 
a  large  reduction  may  be  made  in  the  quantity  of  metal 
used.  The  lateral  bending  resistance  is  also  much  greater 
than  that  of  smooth  bars. 

In  the  Expanded  Metal  system  the  reinforcement  consists  of 
mesh  work  (Fig.  72)  made  by  cutting  slots  in  a  sheet  of  metal 
and  then  pulling  it  transversely  until  a  network  is  formed. 

The  usual  types  of  expanded  steel  used  in  reinforced  concrete 
construction  are  the  3-inch  diamond  mesh,  and  the  rib  mesh, 
a  few  1  J-inch  and  6-inch  diamond  meshes  being  also  occasionally 
used  for  such  work.  The  lighter  weights  of  the  |-inch  and 


FIG.  71.— Indented  Bars. 


EXPANDED  METAL  237 

1  |-inch  diamond  meshes  are  frequently  used  in  concrete  for 
encasing  steelwork. 

The  "  3-inch  diamond  mesh  "  measures  three  inches  by 
eight  inches  from  centre  to  centre  of  its  intersections  or  junc- 
tions. The  size  of  the  mesh  is  constant,  but  according  to  the 
thickness  of  the  sheet  from  which  it  is  made  the  strands  vary 
in  sectional  area,  and  thus  several  weights  are  produced. 
Each  intersection  is  twice  the  sectional  area  of  its  strand,  and 
there  are  eight  strands  per  foot  run  short  way  of  mesh. 

It  is  a  curious  fact  that  tests  prove  that  the  process  of 
expanding  raises  the  elastic  limit  and  increases  the  ultimate 
strength,  thereby  greatly  improving  it. 

In  the  rib  mesh  the  ribs  are 
constant  in  cross-section,  but 
are  spaced  at  varying  centres. 
The  total  area  is  available  for 
reinforcement  in  both  the 
"  diamond  mesh  "  and  "  rib 
mesh." 

The  rib  mesh  (Fig.  73) 
expanded  steel  consists  of  a  FIG.  72 -Expanded  Metal. 

Diamond  Mesh, 
series  of  straight  ribs,  or  mam 

tension  members,  which  in  the  process  of  expansion  are 
left  rigidly  connected  by  light  cross  ties  which  act  as 
spacers.  It  is  essentially  a  bar  reinforcement  without  some 
of  the  disadvantages  of  the  latter,  for  it  is  obviously  an  improve- 
ment on  ordinary  bars  for  slab  reinforcement  to  have  them 
attached  together  as  in  a  sheet  of  rib  mesh  expanded  steel. 
It  is  cut  in  various  meshes,  in  sheets  up  to  twenty-four  feet 
six  inches  long,  which  are  made  from  one  original  section  of 
steel  by  cutting  the  light  cross  ties  shorter  or  longer,  so  as  to 
allow  of  the  ribs  being  opened  out  less  or  more  widely.  While 
rib  mesh  expanded  steel  differs  from  diamond  mesh  expanded 
steel,  in  that  the  meshes  are  square  instead  of  diamond  shape, 
calculations  of  safe  working  loads  for  the  two  materials  are 
based  on  the  same  tensile  strength,  so  that  the  one  mesh  may 
be  substituted  for  the  other  so  long  as  the  same  sectional  area 
is  used. 

The   advantages   of   expanded   steel   as   reinforcement   for 


238  REINFORCED  CONCRETE 

concrete  make  it  specially  useful  for  plain  and  curved  areas. 
It  is  supplied  in  flat  sheets  ready  for  use  ;  it  packs  closely,  and 
is  easily  transported,  and  quickly  handled  ;  it  is  simple, 
economical  and  effective.  The  expanded  sheets  are  machine- 


FIG.  73.— Expanded  Metal,  Rib  Mesh. 

made,  and  although  of  network  formation  there  are  no  loose 
strands,  as  the  junctions  between  the  meshes  remain  uncut 
during  the  process  of  manufacture,  and  thus  the  strands  or 
members  are  all  rigidly  connected,  and  have  continuous  fibres. 


NETTING  REINFORCEMENT  239 

The  meshes  key  into  each  other  and  consequently  interlock 
where  the  sheets  overlap  at  joints,  thus  the  reinforcement  may 
be  made  continuous  no  matter  how  large  the  area  be  treated. 
Excellent  mechanical,  as  well  as  cross  bond,  and  anchorage 
are  obtained  with  expanded  steel,  and  owing  to  its  peculiarity 
it  cannot  slip  within  the  concrete,  for  this  is  most  efficiently 
locked  within  the  meshes. 

The  special  feature  of  expanded  steel  is  that  it  is  a  solid 
sheet  of  network  formation  wherein  all  the  strands  or  members 
are  all  rigidly  connected,  and  when  in  position  cannot  be 
displaced  by  the  laying  and  tamping  of  concrete.  It  is  there- 
fore quite  reliable  as  a  reinforcement  as  the  steel  goes  where 
it  is  planned  to  go  without  requiring  skilful  setting  out  on  the 
part  of  the  workmen. 

In  this  respect  it  is  superior  to  loose  bars  placed  by  measure- 
ment and  tied  at  intersections  with  wire,  for  if  one  bar  is  lower 
than  another  it  is  evident  that  the  former  will  be  more  highly 
stressed  than  the  latter,  and  the  full  value  of  the  total  reinforce- 
ment will  not  be  obtained. 

The  distribution  of  stress  by  expanded  steel  is  such  that 
wherever  a  load  may  come  there  is  steel  to  transmit  it  in  all 
directions,  so  that  a  load  does  not  affect  merely  the  portion  of 
slab  directly  under  it.  When  a  concentrated  load  comes  on 
a  slab  reinforced  with  separate  bars,  only  the  bar  under  the 
load  is  affected  ;  with  the  expanded  steel  the  mesh  distributes 
the  stress  in  all  directions.  Expanded  metal  sheets  are  manu- 
factured in  various  sizes  up  to  sixteen  feet  the  long  way  of 
the  meshes,  and  there  are  some  seventy-six  varieties  with 
meshes  one  and  three  sixths  inches  to  six  inches  wide.  The 
manufacturers  claim  that  expanded  metal  saves  about  75  per 
cent,  of  the  bulk  of  the  concrete  when  used  as  a  tension  bond. 
It  is  specially  used  as  lathing  for  partitions  and  in  floors,  but 
is  equally  available  for  beams,  stanchions  and  columns,  bridge 
work,  reservoirs,  conduits,  sewer  pipes  and  retaining  walls. 
For  beams,  a  square  bar  is  rolled  with  a  lateral  rib  ;  the  latter 
expands  upwards  so  as  to  form  a  mesh  which  takes  the  place  of 
the  stirrups  in  other  systems. 

Richard  Johnson,  Clapham  and  Morris,  Ltd.,  use  a  form  of 
wire  netting  (Fig.  74)  as  reinforcement.  For  many  purposes 


240 


REINFORCED  CONCRETE 


this  is  quite  satisfactory,  but  for  heavy  work  drawn  wire 
network  is  best  avoided. 

Another  form  of  reinforcement  recently  introduced  into 
this  country  is  the  "  triangle  mesh  "  supplied  by  the  United 
States  Products  Co.  It  is  made  of  hard  drawn  cold  steel  wire 
with  an  elastic  limit  of  22  tons  per  square  inch.  There  are 
no  welds  in  this  reinforcement,  which,  being  of  the  hinged 
joint  type,  is  flexible  without  producing  initial  stresses  when 
bent  (Fig.  75). 

A  large  number  of  other  shapes  of  reinforcing  bars  and 
meshes  have  been  devised,  but  the  foregoing  are  sufficient  for 
the  student  to  gain  some  idea  of  the  arrangements  most 
commonly  used. 

The  reinforcement  in  arches  and  arched  bridges  constructed 
of  reinforced  concrete  serves  three  purposes,  namely,  to  take 


FIG.  74. — Johnson's  Netting  Keinforcement. 


compression,  to  resist  tension,  and  prevent  shearing  and 
temperature  cracks.  The  reinforcement  is  designed  in  con- 
formity with  the  modern  theory  of  the  elastic  arch,  and  hinges 
are  often  introduced  so  as  to  ensure  the  line  of  stress  passing 
through  given  points.  The  design  should  prevent  tension  in 
the  arch  ring,  and  the  reinforcement  chiefly  serve  to  assist  in 
the  resistance  of  the  compressive  stresses  ;  consequently,  large 
T-shaped  steel  members  are  frequently  adopted. 

Trussed  bridges  of  various  forms  are  constructed  in  rein- 
forced concrete,  but  no  special  principles  are  introduced  in 
their  design,  the  members  consisting  of  posts,  perpendicular 
or  inclined,  and  beams  of  cantilevers. 

For  bridges,  etc.,  reinforced  concrete  has  one  marked 
advantage  over  most  other  materials,  in  that  it  may  indicate 
conditions  of  maximum  allowable  tension  in  its  embedded 


ARCHES  AND  BRIDGES 


241 


steel  before  actual  danger  exists.  This  advantage  rests  in  the 
fact  that  the  coefficient  of  elasticity  of  the  concrete  and  of 
the  embedded  steel  do  not  bear  the  same  ratio  as  their  allowable 
stresses.  When  the  embedded  steel  is  stressed  to  5,000  Ibs. 
per  square  inch,  invisible  cracks  occur  in  the  surrounding 
concrete.  At  10,000  to  15,000  Ibs.  per  square  inch  these 
cracks  become  visible.  At  20,000  Ibs.  per  square  inch  they 
frequently  become  objectionable. 

In  arch  design,  where  the  stresses  in  the  arch  are  magnified 
by  the  behaviour  of  the  spandrel  walls,  cracking  of  the  concrete 


FIG.  75. — Triangle  Mesh  Eeinforcement. 

serves  the  purpose  of  an  extensometer  to  detect  excessive 
stresses.  If  an  arch  is  designed  too  flat  at  the  crown,  cracks 
will  appear  in  the  spandrel  near  the  ends  of  the  span.  If  too 
flat  at  the  haunches,  cracks  will  appear  in  the  coping  over  the 
crown  or  through  the  arch  ring  at  the  haunches  directly  under 
the  spandrel  only,  and  not  extending  far  into  the  soffit  of  the 
arch.  The  tensile  stress  in  the  arch  itself  is  rarely  sufficient 
to  show  cracks  actually  penetrating  the  arch  ring.  Small 
cracks,  particularly  in  the  spandrels,  are  no  indications  of 
failure,  being  merely  the  magnified  effects  of  movements  in 
the  arch  ring,  but  a  properly  designed  and  erected  arch  will 

C.  B 


242 


REINFORCED  CONCRETE 


be  free  from  such  cracks,  if  provided  with  expansion  joints 
in  the  spandrels  above  each  springing. 

Reinforced  concrete  is  now  extensively  used  in  the  construc- 
tion of  conduits,  water  mains,  and  sewers.  The  latter  are  usually 
constructed  in  situ — that  is,  the  concrete  is  mixed  and  placed 
to  set  in  the  position  it  will  finally  occupy,  although  a  great 
number  of  concrete  sewer  pipes  of  large  diameter  are  manu- 
factured and  sold  as  an  ordinary  market  commodity.  Water 
and  drain  pipes  are  occasionally  constructed  in  situ,  but  are 
usually  made  in  moulds.  The  reinforcement  is  generally  of 
meshwork,  preferably  with  the  warp  of  spiral  form,  the  strands 
perfectly  crossing  the  pipe  at  an  angle  (Fig.  76),  and  not  running 
longitudinally  and  at  right  angles  to  the  length.  The  spiral 
reinforcement  serves  to  resist  the  bursting  pressure.  Expanded 

metal  and  wire 
meshwork  have 
been  largely  used 
for  this  purpose. 
In  the  Bonna 
system,  special 
cruciform  bars  are 
wound  spirally 
inside  and  outside 

a    thin    sheet    of 
FIG.  76.— Coignet  Pipe.  ^^     the      gteel 

serving  to  prevent  contamination  by  penetration  of  external 
moisture  in  the  soil  or  percolation  under  great  pressure. 

After  being  made,  concrete  pipes  must  usually  be  kept  at 
a  temperature  of  50°  F.  or  above,  protected  from  direct  sun- 
light and  air  currents,  for  at  least  seven  days,  during  which 
period  they  must  be  kept  moist  by  sprinkling  with  water. 
They  should  not  be  removed  to  the  open  air  until  they  are  at 
least  a  month  old.  If  the  pipes  are  to  be  "  steam  cured  " 
they  are  placed  in  an  autoclave  and  subjected  to  the  action 
of  moist  steam  for  about  forty-eight  hours,  if  the  steam  is  at 
or  below  212°  F.,  or  for  a  much  shorter  period  if  the  steam  is 
under  considerable  pressure,  as  described  later  under  "  Lime 
Sand  Bricks." 

For  docks,  reservoirs,  aqueducts  and  water  tanks  of  all  sizes, 


PILES 


243 


reinforced  concrete  has  proved  about   15  per  cent,  cheaper 

than  mass  concrete  or  bricks,  and  about  20  per  cent,  cheaper 

than  stones,  and  in  addition  there  are  practically  no  charges 

for     maintenance    or    repair. 

Silos,  magazines  for  explosives, 

coal  bunkers,  as  well  as  tanks 

for  oil,  brine,  and  many  other 

fluids,    are   proving   perfectly 

satisfactory     providing     that 

they    are    well   designed   and 

the  concrete  is  properly  made 

and  placed. 

Piles    are     constructed     of 

concrete  in  a  manner  similar 

to  columns  when  they  are  cast 

in   place,   but   piles  cast  in  a 

mould   are  not  usually  rein- 
forced so  strongly.     Concrete 

piles    are    superior    to    wood 

both  as  regards  strength  and 

durability,     and     are     much 

cheaper  than  piles  constructed 

exclusively  of  steel. 

The  essentials  of  a  good  pile  are  the  following  : — 
(a)  That  it  shall  be  capable  of  being  driven  into  the  ground 

(either   soft    or   hard,    wet    or 
dry)   to  such  a  depth   as  will 
enable    the  buried  portion  to 
support    the    weight    of   some 
superstructure,  or  to  withstand 
I  force  applied   against  it  in   a 
I  horizontal  direction,  or  to  resist 
forces    applied    in    any    other 
desired  direction. 

(6)  That       the       projecting 
portion  of    the    pile   shall    be 

capable  of  supporting  axial  vertical  loads  with  perfect  safety, 

and  of  supporting  eccentric  vertical  loads  without  appreciable 

flexure. 

R2 


FIG.  77. —Overloaded  Column, 
with  Insufficient  Longitudinal 
Reinforcement. 


FIG.  78. 


244 


REINFORCED  CONCRETE 


(c)  That,  if  the  pile  is  to  be  moulded  before  use  and  not 
"  cast  in  place,"  it  shall  possess  rigidity  and  elastic  strength 
sufficient  to  permit  it  to  be  transported,  slung,  and  driven 
without  injury. 

Piles  may  be  circular,  square,  or  any  other  cross-section, 
the  usual  shape  for  foundation  being  that  of  a  square  with 
chamfered  edges,  whilst  rectangular  piles  are  used  for  sheeting. 

The  reinforcement  usually  consists  of  stout  longitudinal  bars 
with  suitable  wire  ties,  the  construction  being  very  similar  to 
that  of  columns,  and  similar  considerations  apply  to  them 
(see  p.  221).  The  metal  exposed  in  cross -section  varies  from 
2  to  5  per  cent.,  and  is  almost  invariably  of  round  rods,  clamped 
and  tied  as  in  the  columns. 

The  following  table  gives  particulars  of  Hennebique  piles 
made  and  used  at  Southampton  :— 


Section  of  Pile 
in  inches. 

Reinforcement. 

Percentage  of 
Steel  in 
Cross-section. 

12      X      12 

4  rods  1|  inches  diameter. 

5 

14      X      14 
15      X      15 
16      X      12 

»      -"-4       "             » 

>»              -*-  U                 »!>                                 ?> 

1  5 

55              X  8                  5>                                  »> 

2i 
21 
4J 

It  is  important  that  the  longitudinal  rods  should  be  of  ample 
diameter,  as  thin  rods  may  prove  disastrous  (Fig.  77). 

As  the  power  required  to  drive  a  pile  is  largely,  if  not  entirely, 
due  to  friction  between  its  surface  and  that  of  the  surrounding 
earth,  a  given  volume  of  concrete  in  the  form  of  a  hollow 
cylinder  should  be  more  effective  than  if  applied  as  a  solid 
cylinder,  or  conversely,  concrete  may  be  saved  by  using  a 
hollow  pile  of  the  same  external  dimensions  as  a  solid  one. 

This  point  is  illustrated  in  Fig.  78,  where  the  left-hand 
diagram  represents  the  cross-section  of  a  13-4  inch  diameter 
solid  cylinder  with  the  area  of  141  square  inches  and  the 
circumference  of  42  inches;  and  the  right-hand  diagram 
represents  the  cross-section  of  an  18-inch  diameter  hollow 
cylinder  with  the  net  area  of  141  square  inches  and  the  circum- 


PILES 


245 


^- — ,-. — JCi. 


ference  of  56-5  inches.     Thus,  for  the  same  area  of  material, 
the  circumference  of  the  hollow  cylinder  is  over  33J  per  cent, 
more  than  the  circumference  of  the  solid  cylinder,  and  the 
bearing  power   of   a   hollow   pile   can  readily  be 
made  one-third  greater  than  that  of  a  solid  pile 
without  increasing  the  cost  in  the  slightest  degree. 
L.  J.  Mouchel  has  therefore  effected  a  notable 
improvement  by  the  construction  of  hollow  piles 
with  a  reinforcement  based  on  the   Hennebique 
system. 

For  practical  reasons  it  is  generally  desirable 
to  employ  piles  of  rectangular  form,  and  for  this 
reason  the  Mouchel  patent  hollow  piles  are 
usually  made  with  the  cross-section  shown  in 
Fig.  79,  thereby  sacrificing  a  small  proportion  of 
the  saving  that  could  be  effected  by 
the  adoption  of  the  circular  form  in 
order  to  gain  advantages  which 
appeal  to  the  engineer  using  piles  as 
structural  members. 

The  head  and  point  of  the  pile  are 
usually  protected  by  steel  caps. 
When  casting  piles  in  place  the  re- 
inforcement is  first  erected  and  is 
surrounded  by  the  necessary  centering 
or  forms.  The  concrete  is  put  in  in 
small  quantities  and  carefully  tamped 
around  the  reinforcement.  It  is  most 
convenient  to  erect  three  sides  of 
the  form  and  to  build  up  the  fourth  as 
the  addition  of  concrete  proceeds. 
After  three  days  the  forms  are 
removed  and  the  pile  is  left  for 
several  weeks  in  order  that  it  may  harden  properly. 
Coignet  Pile.  After  ^his  ^  is  ready  for  driving.  The  piles  must  be 
made  of  exactly  the  required  length  as  it  is  costly 
to  cut  them.  Piles  which  have  been  treated  in  steam  for  three 
days  are  ready  for  driving  at  the  end  of  this  period  ;  the  steam 
or  high  temperature  greatly  increase  the  rate  of  hardening. 


FIG.  79.— 

Mouchel 

Hollow  Pile. 


246 


REINFORCED  CONCRETE 


Chimneys  constructed  of  reinforced  concrete  can  be  erected 
by  well-organised  firms  in  about  half  the  time  and  for  about 
half  the  cost  of  brick  chimneys  of  the  same  size.  The  larger 
the  chimney  the  greater  is  the  saving,  as  a  large  brick  shaft 


FIG.  81. 
Hennebique 
Pile, 


FIG.  82. — Hennebique 
Sheet  Pile. 


FIG.  83.— Pile  with  Solid 
Core. 


CHIMNEYS 


247 


must  have  a  very  thick  base  in  order  to 
provide  the  necessary  support.  The  weight 
and  space  occupied  by  a  concrete  chimney 
are  also  less  than  a  brick  shaft,  this  being 
important  where  the  foundation  is  treacherous, 
and  it  is  also  claimed  that  a  reinforced  con- 
crete chimney,  if  properly  designed,  has  a 
greater  stability  than  one  of  brick  or  stone 
on  account  of  its  monolithic  character.  The 
resistance  of  concrete  to  the  heat  and  abrasive 
action  of  flue  gases  appears  to  meet  all 
requirements,  and  experimental  blocks  of 
concrete  placed  in  brick  chimneys  were  found 
to  have  a  greater  strength  than  similar  blocks 
stored  for  an  equal  time  under  water.  It  is, 
however,  necessary  that  the  concrete  should 
be  well  set  before  being  heated  ;  two  months 
or  more  should  be  allowed  to  lapse  before 
the  chimney  is  used. 

A  temperature  of  520°  C.  is  considered  to  be 
the  highest  safe  temperature  for  large  concrete 
chimneys,  though  most  concrete  can  be  heated 
on  one  side  only  to  900°  C.  for  several  hours 
without  ill  effects.  It  is,  however,  essential 
that  only  sand  be  used  as  aggregate,  as  stone 
"  flies  "  under  the  action  of  heat. 

There  have  been  several  serious  failures  of 
concrete  chimneys,  especially  in  the  United 
States.  In  almost  every  case  these  have 
been  traced  to  faulty  workmanship  in  con- 
struction and  not  to  errors  in  design.  This 
is  due  to  the  use  of  a  safety  factor  of  five 
for  transverse  resistance  and  to  allowing  for 
stresses  set  up  by  a  wind  travelling  at  the 
rate  of  100  miles  per  hour — a  velocity  greater 
than  that  of  a  cyclone.  A  maximum  wind 
pressure  of  50  Ibs.  per  square  foot  on  a  square  FIG.  84.  Con- 
shaft,  and  two-thirds  of  this  on  a  round  one  is  jidere  Pile  with 
i  I-,  bpiral  Rein- 
ample  allowance.  forcement. 


248 


REINFORCED  CONCRETE 


Engine  and  dynamo  beds  and  foundations  for  machinery  are 
more  free  from  vibration  when  made  of  concrete  than  of  brick- 
work or  stone.  The  bed  can  also  be  constructed  to  any  shape 
at  a  cheaper  rate  than  when  other  materials  are  used. 

Foundation  rafts  in  boggy  or  "  quick  "  ground  are  made  of 
reinforced  concrete,  and  thus  enable  large  buildings  to  be 
erected  on  what  would  otherwise  be  too  treacherous  a 
foundation. 

Floors  made  of  concrete  are  usually  constructed  of  iron  girders 
or  concrete  beams  (Fig.  85)  placed  fairly  close  together,  the 
intervening  space  being  occupied  by  slabs  3|  inches  to  6  inches 
thick,  formed  of  a  1:2:5  or  other  suitable  mixture,  and 
reinforced  with  TVinch  rods  placed  4  inches  apart,  or  with 


-Expanded  Metal  Lathing 
to  be  fixed  when  Plastering  is  required 


//$  mesh  Expanded  Steel  - 
for  Concrete  Encasing 


-Reinforced  Concrete  Beam 
Expanded  Sted  Bars 


Rein  forced  Concrete  Beam 
Reinforcing  Rods 


FIG.  85. — Use  of  Expanded  Metal  for  Beams  and  Floors. 

network  reinforcement  so  that  the  whole  structure  can  carry 
a  load  of  3  cwt.  per  square  foot  in  addition  to  the  weight  of 
the  floor  itself  for  factories,  or  1|  cwt.  per  square  foot  for 
houses  and  public  buildings.  Many  designs  of  floors  are  in 
use,  some  of  them  for  very  large  spans. 

Paving  blocks  and  floor  slabs  made  of  concrete  are  used 
increasingly  in  districts  where  natural  stone  is  costly.  Curbs 
for  pavements  are  cheaper  when  made  of  concrete  than  of 
sandstone,  and  are  equally  durable. 

Building  blocks  of  concrete  are  also  made  in  large  quantities, 
the  claim  being  made  that  as  a  builder  can  lay  fifty  blocks 
2  feet  X  1  foot  X  9  inches  per  day,  the  use  of  such  blocks  is 
cheaper  than  ordinary  mass  concrete.  The  plastic  concrete  is 


FLOORS  AND  BLOCKS 


249 


placed  in  moulds  (Figs.  87,  88)  of  cast  iron  or  mild  steel, 
and  is  gently  tamped  until  the  surplus  water  rises  to  the 
surface.  After  being  left  for  a  short  time  until  the  cement 
has  set,  the  sides  of  the  mould  are  allowed  to  fall  and  the 
block  or  slab  is  removed  and  stored  in  a  cool  shed  until  it  is 
hard  enough  for 
use.  The  square 
slabs  used  for 
paving  are  fre- 
quently made  in 
a  strong  wooden  1 
or  metal  frame, 
which  is  laid  on 
a  piece  of 
matting  or 
canvas  on  a 
smooth  level 
floor.  The  con- 
crete is  placed 
in  this  frame, 
tamped  care- 
fully, and  the 
surplus  material 
removed  with  a 
long  -  bladed 
knife.  After 
the  cement  has 
set,  the  frame 

is  removed  leaving  the  paving  block  on  the  canvas.  If  a 
sufficiently  fine  aggregate  and  a  fairly  rich  mixture  is  used 
the  frame  may  usually  be  lifted  off  as  soon  as  the  tamping 
is  finished.  The  canvas  gives  a  pleasing  texture  to  the  surface 
of  the  slab  and  renders  it  non-slipping.  Such  slabs  are  not 
reinforced  in  the  ordinary  sense  of  the  term,  the  metal  (if  any) 
embedded  in  them  being  intended  to  prevent  them  spalling, 
and  not  to  increase  their  tensile  strength. 

When  the  number  of  blocks  or  slabs  is  sufficiently  large  a 
machine  is  desirable.  Care  should  be  taken  in  selecting  one 
for  this  purpose,  as  some  of  those  on  the  market  are  far  from 


FIG.  86. — Concrete  Building  Blocks. 


250 


REINFORCED  CONCRETE 


satisfactory.  Among  the  best  is  the  "  Winget  "  concrete  block 
machine,  which  consists  of  a  mould  box  with  hinged  sides  and 
ends  carried  in  the  frame  hung  in  trunnions,  as  shown,  with  core 
blocks  set  inside.  When  the  lever  at  the  right-hand  side  of  the 
illustration  is  pulled  down  the  bottom  of  the  mould  box  (which 
is  formed  with  a  loose  pallet  inserted  for  each  block)  is  lifted  up, 
and  at  the  same  time  the  sides  of  the  box  fall  outward,  leaving 
the  finished  block  on  the  pallet  ready  to  be  carried  off. 
When  it  is  required  to  use  the  machine  in  the  face-down 

position  the  frame 
carrying  the  mould  box 
is  swung  over  in  the 
trunnions  into  a  hori- 
zontal position  with  the 
same  lever  on  releasing 
the  stop  controlled  by 
the  small  horizontal 
lever  at  the  right-hand 
end  of  the  machine. 
The  cores  are  carried  on 
a  fixed  bed  plate  under 
the  pallet  forming  the 
bottom  of  the  mould 
box. 

The  movements  neces- 
sary to  make  a  block 
are  the  fewest  and 
simplest  possible,  so  that  the  maximum  output  is  secured 
with  a  minimum  cost  for  labour.  If  the  instructions  given 
with  the  machine  are  followed,  two  men  can  make  about  400 
blocks  or  800  slabs  per  day. 

Concrete  roads  are  usually  made  of  mass  concrete  without 
reinforcement,  except  for  the  paving  curbs,  but  there  is  a 
movement  in  favour  of  making  the  whole  road  of  reinforced 
concrete,  and  using  the  hollow  space  beneath  it  for  sewers, 
gas  pipes,  etc. 

Lintels  have  steel  joist  bars  or  rods  embedded  in  them,  but 
the  distribution  of  these  varies  greatly  in  different  cases. 
A  1:2:4  mixture  is  generally  employed. 


FIG.  87. — Mould  for  Slabs. 


STAIRS  AND  STEPS 


Tiles  made  of  cement  are  used  for  both  roofing  and  flooring. 
They  are  made  in  metal  moulds  consisting  of  a  lower  box  or 
container  into  which  an  upper  plate  or  plunger  fits  closely. 
The  plunger  is  lifted,  its  lower  surface  and  the  inside  of  the  box 
are  oiled  or  wetted,  the  box  is  filled  with  concrete  paste,  and 


FIG.  88. — "  Winget  "  Block-making  Machine. 

the  plunger  is  brought  down  so  as  to  compress  the  mixture, 
the  tile  is  turned  out  on  to  a  bench,  and  in  a  few  hours  is  ready 
for  use.  About  500  tiles  can  be  made  in  a  day  by  one  man. 

Stairways  and  steps  made  on  the  site  are  reinforced  longi- 
tudinally in  a  manner  similar  to  beams  and  transversely  near 
the  front  of  the  tread,  with  vertically  placed  studs  in  order  to 


252 


REINFORCED  CONCRETE 


reduce  the  tendency  of  this  portion  of  the  steps  to  spall  or 
break  away  under  traffic.  The  treads  and  risers  are  also  cast 
solid,  and  sold  ready  to  be  fixed  as  stone  stairs. 

Railway  sleepers  are  used  in  large  numbers  in  the  United 
States.  Though  slightly  more  costly  than  wood  in  the  first 
instance,  they  are  reported  to  be  more  durable,  and  so  become 
more  economical. 

Telegraph  poles  and  tramway  standards  are  cheaper  in  concrete 
than  in  steel.  They  are  of  tubular  form,  and  must  be  carefully 
made  in  such  a  manner  that  the  reinforcement  does  not  slip 
during  the  tamping. 


N°l  EM  Lathing 


f\l°  92  Expanded  Metal  Lathing 


FIG.  89. — Use  of  Expanded  Metal  in  Stairs. 


Pit  props,  gate  posts,  boats,  pontoons,  garden  ornaments,  and 
many  other  useful  and  ornamental  articles  are  also  made  of 
concrete.  In  fact,  new  uses  of  this  remarkable  substance  are 
constantly  being  found.  Some  of  these  uses  are  fantastic,  in 
others  the  naturally  suitable  materials  are  discarded  in  favour 
of  concrete,  merely  because  of  a  desire  to  use  the  latter  material, 
but  in  the  majority  of  cases  concrete  may  rightly  be  used  in 
situations  where  other  materials  are  less  readily  available,  and 
in  circumstances  where  concrete  is  particularly  suitable. 

In  all  concrete  work  it  is  essential  that  ample  precautions 
shall  be  taken  to  secure  the  concrete  being  properly  made  and 
placed,  to  prevent  the  forms  being  taken  away  too  hurriedly. 


CAUSES  OF  FAILURE  253 

and  to  ensure  that  no  pains  are  spared  to  keep  the  surface  of 
the  material  properly  wetted  when  such  treatment  is  necessary. 
The  repeated  failures  of  concrete  construction  have,  in  almost 
every  case,  been  due  to  the  use  of  concrete  under  unsuitable 
conditions,  or  to  the  improper  manipulation  of  the  materials 
and  forms  rather  than  to  inherent  disability  in  the  design  of 
the  structures.  Fortunately,  concrete  structural  work  tends 
increasingly  to  become  a  separate  occupation,  and  this  will 
reduce  the  risks  of  its  use,  and  men  will  work  more  and  more 
skilfully  as  they  become  accustomed  to  the  material.  The 
simplicity  of  many  forms  of  concrete  construction  makes  it 
specially  attractive  to  farmers  and  others  in  isolated  situations, 
but  the  risks  of  imperfect  mixing,  etc.,  then  become  more 
serious.  Concrete  is  an  invaluable  material,  yet  those  who  see 
its  advantages,  but  not  its  drawbacks,  may  easily  find  them- 
selves and  others  seriously  inconvenienced,  especially  in  the 
case  of  buildings  erected  by  men  unskilled  in  this  branch 
of  work. 

At  the  same  time,  even  among  the  most  skilled  workers  there 
is  still  room  for  much  improvement  in  the  preparation  of 
concrete,  particularly  with  regard  to  its  strength  and  permea- 
bility. Grading  the  aggregate  more  carefully  and  into  a 
larger  number  of  portions  than  is  at  present  customary,  will 
probably  prove  to  be  the  most  efficacious,  but  the  cost  of  this 
increased  grading  is  so  frequently  found  to  be  greater  than  that 
of  an  additional  quantity  of  cement  that  it  is  difficult  to  increase 
the  number  of  grades  above  that  now  in  use.  The  study  of 
the  proportion  of  voids  in  the  concrete  and  of  the  materials 
composing  it  will  naturally  give  the  clue  to  obtaining  stronger 
and  more  impermeable  concretes,  and  will  not  improbably 
result  in  a  decrease  in  the  use  of  waterproofing  materials. 
That  the  concrete  of  the  future  will  be  superior  to  that  of  the 
past  cannot  be  doubted  by  those  who  are  closely  in  touch  with 
the  many  attempts  now  being  made  to  increase  the  stringency 
of  the  present  standard  specifications. 


CHAPTER  IX 

SPECIAL   PROPERTIES    OF   CONCRETE 

THE  properties  of  concrete  vary  with  its  age  and  composition 
as  well  as  with  the  quality  of  the  cement  used. 

The  crushing  strength  of  concrete  is  roughly  proportional  to 
the  amount  of  cement  in  the  material,  the  maximum  strength 
being  reached  when  all  the  voids  are  filled  and  each  particle 
of  material  is  coated  with  cement.  As  explained  in  a  previous 
chapter,  this  usually  corresponds  to  a  weight  of  cement  equal 
to  ii  times  the  amount  of  voids  in  the  quantity  of  sand 
used  in  a  batch  of  concrete.  It  is  not  usually  wise  to  employ 
a  concrete  with  a  compressive  strength  after  twenty-eight 
days  which  is  below  2,400lbs.  per  square  inch.  (For  further 
particulars  see  p.  126.) 

The  tensile  strength  of  concrete  may  be  tested  in  the  same  way 
as  that  of  cement  (p.  128),  but  is  usually  assumed  to  be  one- 
tenth  that  of  the  crushing  strength. 

The  shear  strength  of  concrete  is  not  known  with  accuracy, 
and  appears  to  depend  largely  on  the  nature  of  the  aggregate. 
Professor  A.  W.  Talbot  finds  it  to  be  about  half  the  compressive 
strength,  but  one-tenth  of  the  latter  is  the  figure  usually 
assumed. 

The  effect  of  testing  concrete  in  shear  and  also  in  lines  of 
fracture,  are  shown  in  Fig.  90.  The  blocks  tested  were 
9  inches  X  12  inches  X  12  inches.  The  result  obtained  in  the  first 
case  is  very  low  and  possibly  due  to  some  latent  imperfection. 

The  transverse  strength  of  concrete  has  also  been  tested 
in  certain  instances.  The  results  obtained,  after  reduction  to 
their  equivalent  values,  for  a  beam  twelve  inches  square  in 
cross-section,  and  twelve  inches  long  between  supports,  are 
exhibited  in  Fig.  91.  The  weight  given  is  the  breaking  load 
applied  centrally. 

Shear  tests  are  peculiarly  difficult  on  account  of  the  material 
bending  instead  of  simply  shearing,  and  it  is  not  impossible 
that  future  experiments  will  give  even  higher  results. 


ADHESION 


255 


The  adhesion  between  concrete  and  steel  is  remarkably  great, 
so  that  there  need  be  no  fear  of  any  slipping  occurring  so  long 
as  the  reinforcement  is  properly  designed.  It  is  upon  this 
adhesion  that  much  of  the  value  of  reinforcement  depends. 


«.-  9" 


10  Tons 


12s  Tons 


2  to  1 


2  to  ) 


FIG.  99. — Shearing  Strength  of  Concrete  and^Nature  of  Fracture 
under  Shearing  Stress.     (H.  E.  'Jones.) 

The  adhesion  of  concrete  to  steel  cannot  alone  be  depended 
upon  to  transmit  the  stresses  from  the  reinforcement  to  the 
surrounding  concrete. 

When  the  surface  of  the  steel  is  smooth  and  free  from  obvious 
indentations  or  projections,  all  adhesion  must  be  due  to  the 
cement  particles  enter- 
ing the  microscopical 
irregularities  on  the 
surface  of  the  metal, 
so  that  whilst  the 
adhesion,  under  such 
circumstances,  is  often 
remarkable  it  may  be 
greatly  increased  by 


20 


i    10 


Proportion  of  Aggregate  to  Matrix 

FIG.  91. — Transverse  Strength  of 
Concrete.     (H.  E.  Jones.) 


the    use    of     a    more 

irregular   surface. 

Moreover,  Professor   Popplewell   has    shown   that  when  steel 

bars   are   under    stress    the    reduction   in    area    which    they 


256          SPECIAL  PROPERTIES  OF  CONCRETE 

suffer  destroys  the  adhesion  if  the  bars  of  the  steel  are 
smooth,  whilst  the  bar  with  ridges,  indentations  or  other 
irregularities  of  surface  retains  its  grip  on  the  concrete 
until  the  latter  is  actually  broken.  Vibration  of  the  concrete 
mass,  as  a  whole,  also  reduces  adhesion,  and  may  be  serious 
if  completely  smooth  bars  with  no  shear  members  are 
used.  It  is  now  well  known  that  the  shocks  and  vibration 
inseparable  from  the  average  building  are  sufficient  to  reduce 
appreciably  and  sometimes  to  destroy  the  adhesion  when 
plain  rods  or  bars  are  used.  To  guard  against  this  risk 
many  devices  are  employed  by  engineers  who  use  plain  bars, 
such  as  splitting  and  bending  the  bars  at  their  ends  to  obtain 
anchorage.  Such  methods  are  not  wholly  satisfactory,  and 
are  often  inadequate,  and  many  types  of  deformed  bars  have 
been  devised  to  overcome  these  objections.  For  some  purposes 
there  can  be  no  doubt  that  the  best  form  of  reinforcement  is 
a  rigid  network  giving  a  dependable  mechanical  and  cross 
bond  ;  for  others,  indented  or  ridged  bars  are  preferable. 

Adhesion  is  measured  by  embedding  a  steel  rod  in  a  rectan- 
gular block  of  concrete  and,  after  a  suitable  time,  measuring 
the  force  required  to  pull  it  out.  The  area  of  the  surface  of 
the  embedded  bar  must  also  be  ascertained.  The  Joint 
Committee  under  the  auspices  of  the  Royal  Institute  of  British 
Architects  recommends  a  working  adhesive  stress  of  100  Ibs. 
per  square  inch,  though  the  actual  force  required  to  separate 
a  rod  from  the  concrete  in  which  it  is  embedded  varies  from 
550  to  650  Ibs.  per  square  inch. 

The  adhesion  is  increased  by  coating  the  steel  reinforcement 
before  use  with  a  thin  slurry  made  of  cement  and  water,  and 
by  employing  ribbed  or  corrugated  reinforcement  bars  instead 
of  smooth  ones. 

For  information  concerning  the  loads  which  can  be  carried 
by  reinforced  concrete  see  p.  213. 

In  addition  to  its  strength,  many  of  the  advantages  of 
concrete  as  a  structural  material  depend  on  certain  of  its 
properties,  such  as  resistance  to  fire,  sea  water,  shocks,  etc., 
and  it  is  therefore  desirable  to  mention  these  special  properties 
in  somewhat  greater  detail. 

Fire  Resistance. — The  behaviour  of  concrete  buildings  when 


FIRE  RESISTANCE  257 

subject  to  fire  has  been  the  subject  of  numerous  investigations 
and  tests.  Small  changes  in  temperature,  such  as  occur  daily 
in  tropical  climates,  cause  a  superficial  cracking  of  the  concrete, 
which  is  of  no  structural  importance  except  as  regards  the 
appearance  of  the  building.  Much  greater  heat,  as  when  a 
structure  is  "on  fire,"  effects  several  changes,  the  chief  of 
which  are  (1)  the  expansion  of  the  material,  which  may 
endanger  the  stability  of  the  structure  ;  (2)  chemical  dehydra- 
tion ;  and  (3)  destruction  of  the  cement  and  aggregates.  All 
steel  structures  tend  to  expand  when  heated,  and  this  has,  in 
the  past,  resulted  in  much  loss  of  property  as  the  girders  have 
pushed  out  walls  in  consequence  of  the  expansion  of  the  metal. 
In  reinforced  concrete  the  steel  is  so  covered  with  concrete — 
which  has  a  low  heat  conductivity — that  its  expansion  is 
reduced  to  a  minimum,  but  the  durability  of  the  material 
during  a  conflagration  is  entirely  dependent  on  there  being  a 
sufficient  thickness  of  concrete  around  the  metallic  reinforce- 
ment. The  opinion  expressed  by  the  R.I.B.A.  Committee 
that  "  Usually  a  cover  of  \  inch  on  slabs,  or  1  inch  on  beams, 
is  sufficient,"  is  much  too  low  where  the  fire  is  likely  to  be  of 
long  duration.  The  tests  of  the  British  Fire  Prevention 
Committee  have  shown  that  2  inches  of  concrete  is  much  more 
likely  to  prove  a  safe  minimum,  with  2 \  inches  for  columns,  and 
3  inches  for  beams.  If  the  steel  is  thinly  covered  or  is  exposed 
at  any  part  of  the  heated  structure,  its  temperature  will  rise 
so  rapidly  that  the  concrete  itself  may  be  strained  sufficiently 
to  cause  it  to  rupture.  This  caused  the  destruction  of  several 
concrete  buildings  in  the  San  Francisco  and  Messina  disasters. 

The  action  of  heat  on  neat  Portland  cement  is  such  that  at 
temperatures  of  300°  or  above,  the  cement  is  dehydrated  and 
rapidly  crumbles  to  powder.  In  well-made  concrete  structures, 
however,  the  heat-conductivity  is  so  low  that  this  integration 
is  confined  to  the  surface  unless  an  unsuitable  aggregate  has 
been  used.  Hence  the  disintegration  of  concrete  by  fire  is  of 
academic  rather  than  practical  importance,  few  conflagrations 
lasting  sufficiently  long  for  serious  destruction  of  the  concrete 
to  occur  by  the  direct  action  of  the  heat. 

Unsuitable  aggregates  greatly  reduce  the  resistance  of 
concrete  to  fire.  The  igneous  rocks  form  the  most  resistant 

c.  s 


258          SPECIAL  PROPERTIES  OF  CONCRETE 

aggregates,  then  broken  bricks,  sandstones,  gravel,  limestone, 
and  furnace  slag.  Ballast — especially  if  containing  flint 
pebbles — cracks  badly  on  being  heated.  Coke  and  cinders  are 
the  least  satisfactory  so  far  as  resistance  to  fire  is  concerned. 

Measurements  of  the  heat  conductivities  of  various  aggregates 
made  by  Professor  Woolson  have  shown  that  gravel  has  a 
high  conductivity,  limestone  comes  next,  then  igneous  rocks 
and  coke  breeze  come  last  with  the  lowest  power  of  heat 
transmission.  Care  must,  however,  be  taken  that  the  breeze  is 
almost  free  from  sulphur.  If  pan  breeze  or  cinders  be 
substituted,  destruction  of  the  concrete  through  oxidation  of 
the  sulphur  compounds  is  likely  to  occur  (p.  151).  The  coke 
must  not  contain  more  than  5  per  cent,  of  bituminous  matter 
or  it  may,  when  thoroughly  heated,  continue  to  burn  of  its 
own  accord. 

The  heat  conductivity  of  concrete  made  with  sandstone  as 
aggregate  has  been  found  by  Professor  C.  L.  Norton  to  be 
•0021  to  -0029  calories  per  square  centimetre  per  centimetre 
per  second  per  degree  C..  or  150  to  200  B.T.U.  per  degree  F. 
per  square  foot  per  inch  thick  per  twenty-four  hours. 
Untamped  and  coke  breeze  concrete  give  about  half  the  above 
results  on  account  of  their  lower  density. 

Investigations  of  concrete  structures  after  severe  conflagra- 
tions show  that  whilst  limestone,  sandstone  and  gravel  aggre- 
gates suffer  under  the  action  of  severe  fires  and  must  be  renewed, 
yet  coke  breeze  is  frequently  only  injured  superficially  and 
does  not  lose  strength.  Unexpected  results  of  this  kind  make 
it  difficult  to  decide  which  is  the  most  suitable  aggregate  to 
resist  fire. 

The  stability  of  the  structure  during  conflagration  is  also 
increased  by  connecting  the  reinforcement  on  the  lower  side 
of  beams  or  floor  slabs  to  that  on  the  upper  sides,  as  the  latter 
are  seldom  injured  by  fire. 

In  buildings  more  than  usually  liable  to  conflagration,  the 
walls,  ceilings  and  floors  should  be  covered  with  a  fire-resisting 
plaster,  as  this,  if  injured,  can  be  cheaply  renewed  ;  terra-cotta 
facings  may  be  used  with  equal  success  though  they  are  more 
costly.  The  terra-cotta  must  be  suitable  for  the  purpose,  as 
if  it  is  dense  in  texture  it  will  fly  to  pieces  when  heated  and  will 


FIRE  RESISTANCE 


259 


prove  quite  unreliable.  Only  porous  terra-cotta  should  be 
employed,  and  the  pieces  should  be  of  moderate  thickness. 
Much  of  the  "  hollow  tile  "  used  in  the  United  States  has  failed 
under  fire  because  of  its  excessively  thin  web.  The  most 
serious  disadvantage  of  terra-cotta  for  protecting  steel  work  is 
the  difference  in  expansibility  of  the  two  materials,  whereby 
the  steel  not  infrequently  becomes  exposed,  particularly  if, 
under  the  combined  action  of  fire  and  a  high  wind,  the  terra- 
cotta cracks  or  falls  away. 

In  considering  the  action  of  fire  on  concrete,  that  of  the  water 
applied  to  quench  the  fire  must  not  be  overlooked.  This  is 
frequently  more  severe  than  that  of  the  fire  itself,  as  the  hot 
concrete,  when  suddenly  quenched,  is  extremely  liable  to 
crack  and  spall. 

The  British  Fire  Prevention  Committee  has  proposed  the 
following  standard  requirement  for  floors  intended  to  resist  fire, 
and  grants  three  groups  of  certificate  according  as  the  protection 
afforded  by  the  material  is  "  temporary,"  "  partial,"  or  "  full." 

» 
STANDARD  CLASSIFICATION  FOR  FLOORS. 


Protection 

Least  Dura- 
tion of  Test. 

Minimum 

Temperature 
of  Fire  to  be 
reached 
during  Test. 

Minimum 
Load  per 
Superficial 
Foot  Dis- 
tributed. 

Minimum 
Superficial 
Area  under 
Test. 

Minimum 
Time  for 
Application 
of  Water 
under 
Pressure. 

Temporary^; 

45  min. 
60      , 

1500°  F. 
1500°  F. 

Optional. 

100  sq.  ft. 
200 

2  min. 

t  Class  A  . 

90      , 

1800°  F. 

112  Ibs. 

100 

al     •  *  (  Class  B. 

120      , 

1800°  F. 

168    „ 

200 

„  n              (Class  A. 

150      . 

1800°  F. 

224    „ 

100 

•\ClassB. 

240      , 

1800°  F. 

280    „ 

200 

Closeness  of  surface  texture  is  often  an  important  factor  in 
determining  the  heat-resistance  of  a  concrete  structure. 
Hence,  concretes  containing  a  larger  proportion  of  cement  will 
usually  prove  better  than  leaner  ones,  a  concrete  made  from  a 
wet  mixture  will  prove  more  resistant  than  one  made  with  less 
water,  and  a  fine  aggregate  will  prove  better  than  a  coarser 
one.  In  this  connection  it  is  important  to  note  that  concrete 
surfaces  which  have  been  treated  with  fluid  water-glass  mixed 

S  2 


260          SPECIAL  PROPERTIES  OF  CONCRETE 

with  three  times  its  weight  of  water,  the  treatment  being 
repeated  after  twenty-four  hours,  appear  to  have  a  much  greater 
resistance  to  fire  than  the  same  concrete  which  has  not  been 
so  treated,  as  the  siliceous  coating  greatly  reduces  the  rate  of 
dehydration. 

Sharp  corners  in  concrete  tend  to  be  more  seriously  affected 
by  fire  than  those  which  are  rounded,  as  the  sharp  angles 
break  or  spall  off  more  readily. 

Resistance  to  fire  is  of  minor  importance  in  some  buildings, 
whilst  it  is  highly  essential  in  others,  so  that  different  forms  of 
concrete  must  be  used  to  meet  different  requirements.  This 
is  the  more  necessary  as  a  floor  which  affords  full  protection 
against  fire  may  not  be  strong  enough  to  meet  the  requirements 
of  a  warehouse  in  which  the  risk  of  damage  by  fire  is  trifling. 

Professor  C.  L.  Norton  has  found  that  a  reinforced  concrete 
beam  6  x  6  x  48  inches,  when  heated  for  an  hour  in  a  fire 
which  was  hot  enough  to  fuse  the  surface,  broke  under  a 
compression  load  of  2,750  Ibs.  A  similar  beam,  similarly 
heated  for  two  hours,  broke  under  a  load  of  1,950  Ibs.,  and  a 
third  beam  which  was  not  heated  at  all  broke  under  a  load  of 
5,700  Ibs.  He  also  found  that  larger  beams  are  weakened  in 
a  lesser  proportion.  Other  experiments  confirm  the  above, 
and  show  that  concrete  still  possesses  a  notable  degree  of 
strength  even  after  four  hours  continuous  exposure  to  a 
temperature  of  about  1,600°  C. 

The  fact  should  never  be  overlooked  that  no  building  is 
absolutely  fire-proof,  and  the  use  of  such  a  term  is  liable  to 
lead  to  unnecessary  carelessness.  The  best  that  can  be  done 
is  to  make  a  structure  as  fire-resisting  as  possible,  and  to  arrange 
a  system  of  alarms  so  that  the  fire  brigade  can  be  on  the  spot 
before  a  serious  conflagration  is  produced. 

Resistance  to  Shocks. — Next  in  importance  to  resistance  to 
fire  comes  the  ability  of  a  concrete  structure  to  withstand 
repeated  shocks.  Such  shocks  need  not  necessarily  be  severe, 
for  comparatively  small  ones  of  great  frequency  will  destroy 
some  structures.  Earthquake  tremors  of  moderate  dimensions 
are  usually  resisted  by  large  monolithic  buildings,  and  in 
countries  liable  to  this  form  of  disturbance  reifnorced  concrete 
is  particularly  valuable.  Most  of  the  shock-resisting  structures 


RESISTANCE  TO  SHOCKS  261 

are,  however,  erected  to  withstand  the  vibration  of  traffic  over 
a  bridge  or  of  machinery  in  a  factory. 

The  ability  of  reinforced  concrete  to  resist  repeated  and 
powerful  shocks  is  shown  in  the  thousands  of  concrete  piles 
which  have  been  successfully  driven  into  moderately  hard 
strata.  In  such  cases  the  weight  of  the  pile  driven  is  usually 
between  two  and  three  tons,  and  the  blows  are  repeated  so 
frequently  as  to  keep  the  pile  always  moving. 

The  ability  of  concrete  structures  to  resist  such  shocks 
depends  primarily  on  the  adhesion  between  the  concrete  and 
the  steel  reinforcement,  and  on  the  suitability  of  the  design 
of  the  reinforcing  members.  Apart  from  these  no  special 
precautions  are  necessary.  The  extensive  use  of  piles  made  of 
reinforced  concrete  is  conclusive  evidence  of  the  ability  of 
this  material  to  resist  shocks. 

Permeability — frequently,  but  quite  erroneously  spoken  of  as 
"  porosity  " — is  the  power  possessed  by  a  substance  to  allow 
water  or  other  fluid  to  penetrate  through  it.  Porosity  consists 
in  the  possession  of  pores  or  voids,  and  on  immersing  a  porous 
substance  in  water  the  pores  will  become  more  or  less  filled 
and  a  corresponding  quantity  of  water  absorbed.  This  is, 
however,  an  entirely  different  property  from  the  permeability 
of  a  material,  such  as  a  roofing  tile  or  slab  of  concrete  in  which 
water  dropped  on  one  side  will  gradually  percolate  through  the 
material  and  appear  in  the  form  of  drops  on  the  other  side. 
Permeability  appears  to  have  no  definite  relationship  to 
porosity,  and  some  concretes  of  low  porosity  are  far  more 
permeable  than  others.  The  amount  of  permeability  depends, 
in  fact,  on  the  number  and  area  of  the  passages  through  the 
material,  and  not  on  the  total  volume  of  pores. 

Most  concrete  is  slightly  porous,  but  if  its  constituents  have 
been  well  graded  and  proportioned,  concrete  should  be  prac- 
tically impermeable. 

Permeability  is  a  serious  defect  in  concrete  intended  for  water 
tanks,  reservoir  and  marine  embankments,  etc.  Hence,  for 
such  purposes  it  is  necessary  to  take  special  care  in  grading  and 
proportioning  the  aggregate,  sand,  cement  and  water, 
and  for  some  purposes  other  materials  are  added  to  make 
the  material  waterproof.  The  methods  for  doing  this 


262          SPECIAL  PROPERTIES  OF  CONCRETE 

and   the   principles    which   underlie  them    are   described    on 
pp.  197—205. 

Resistance  to  Corrosion. — It  is  one  of  the  curious  properties 
of  steel  that  when  embedded  in  concrete  it  does  not  rust. 
Indeed,  a  thin  coating  of  rust  which  may  exist  when  the  steel 
is  embedded  will  be  found,  after  a  time,  to  have  disappeared 
completely.  This  is  generally  explained  as  being  due  to  the 
iron  oxide  comprising  the  rust  combining  with  the  lime  set 
free  during  the  setting  of  the  cement  and  forming  a  hydrated 
calcium  ferrite  which  acts  as  a  protecting  agent. 

The  following  instances  investigated  by  R.  G.  Clark 
show  in  a  striking  manner  the  highly  protective  action  of 
concrete. 

On  the  river  Thames  a  reinforced  concrete  pile  had  to  be 
withdrawn  as  the  result  of  a  very  severe  collision.  The  pile 
had  been  driven  about  three  years  to  a  very  hard  set,  as  it 
carried  a  heavy  load.  After  being  pulled  up,  the  pile  was  laid 
on  the  bank  for  inspection.  At  various  places  along  the  length 
of  the  pile  the  concrete  was  cut  away  and  the  steel  exposed. 
In  each  case  the  steel  had  not  the  slightest  signs  of  rust.  On 
a  pier  further  down  the  same  river  a  reinforced  concrete  tie- 
beam,  that  had  been  in  position  about  eighteen  months,  had 
to  be  cut  away  to  make  provision  for  a  diagonal  brace.  The 
tie-beam  was  midway  between  high  and  low  water  mark,  so 
that  it  had  a  severe  test,  being  alternately  wet  and  dry.  On 
examination  the  steelwork  was  found  as  good  as  when  put  in, 
and  the  original  rust  had  disappeared.  It  might  easily  be 
imagined  in  what  condition  the  steel  would  have  been  if  it 
had  not  been  protected  by  the  concrete.  Again,  at  Southamp- 
ton in  1898,  several  pile  heads  were  cut  off  and  thrown  on  the 
foreshore,  so  that  they  were  alternately  exposed  to  the  air  and 
covered  by  the  tide.  They  were  examined  some  seven  years 
after  by  several  well-known  engineers,  who  found  that  the 
exposed  steelwork  had  greatly  rusted  and  deteriorated,  whereas 
by  chipping  away  the  concrete  the  bars  which  were  embedded 
in  the  concrete  were  found  to  be  as  free  from  rust  as  on  the 
day  when  they  were  first  used. 

The  resistance  of  concrete  itself  to  the  corrosive  action  of 
acids  is  quite  a  different  matter.  Most  acids — both  mineral 


RESISTANCE  TO  CORROSION  263 

and  organic — decompose  Portland  cement  and  therefore  bring 
about  the  destruction  of  any  concrete  to  which  they  may  be 
exposed.  Such  corrosion  is  particularly  noticeable  in  drain 
and  sewerage  pipes,  especially  where  the  sewage  is  obtained 
from  chemical  and  other  factories.  Sometimes  the  acid  does 
not  occur  in  the  original  fluid,  but  is  produced  by  the  oxidation 
of  sulphur  compounds,  etc. 

Coating  the  concrete  with  tar  or  other  acid-proof  material 
is  the  only  method  of  preventing  corrosion  by  acid,  though  the 
impregnation  of  the  concrete  with  some  fatty  or  oleaginous 
substance  is  sometimes  of  assistance.  The  only  really  satis- 
factory "  remedy  "  consists  in  facing  the  concrete  with  an 
entirely  acid-proof  material,  or  in  replacing  it  by  acid-proof  ware 
as  salt-glazed  pipes. 

The  occasional  absorption  of  trifling  quantities  of  very 
dilute  acids  is  less  serious,  and  is  frequently  neglected. 

Fresh  sewage  is,  ordinarily,  alkaline  and  without  action  on 
concrete  ;  it  is  only  when  the  waste  acid  liquors  from  factories 
and  trade  effluents  are  admitted  that  sewage  becomes  corrosive. 
Only  two  courses  are  then  open  :  (a)  to  neutralise  the  sewage 
before  admitting  it  to  the  sewers,  or  (b)  to  construct  the  sewers 
of  a  different  material.  Acid-proof  paints,  such  as  coal  tar, 
are  only  of  slight  value,  as  they  are  soon  worn  away  by  the 
abrasive  action  of  the  flowing  fluids. 

Sewage  which  has  been  treated  in  bacteria  beds,  on  the 
contrary,  is  very  liable  to  develop  acid  sulphur  compounds, 
which  corrode  the  concrete,  particularly  at  the  mean  water- 
level.  This  corrosion  may  usually  be  prevented  by  coating 
the  pipes  with  a  coal-tar  preparation,  but  this  must  be 
frequently  inspected  and  renewed  as  occasion  requires. 

The  admission  of  hot  water  to  sewerage  appears  to  increase 
the  production  of  sulphuretted  hydrogen  and  sulphur  dioxide, 
both  of  which  have  a  marked  deleterious  effect  on  concrete. 
Wine  musts  and  some  other  liquids  of  an  acid  character 
appear  to  be  without  action  on  concrete,  but  Rohland  has 
shown  that  beer  attacks  concrete  to  a  serious  extent. 

Even  fresh  water  containing  an  appreciable  amount  of  carbon 
dioxide  in  solution  will,  in  time,  bring  about  the  destruction 
of  concrete,  The  carbon  dioxide  removes  the  lime  from  the 


264          SPECIAL  PROPERTIES  OF  CONCRETE 

concrete  and  so  reduces  its  strength  and  increases  its  porosity 
at  the  same  time. 

Quite  pure  water  will  also,  in  time,  remove  an  appreciable 
quantity  of  lime  from  concrete,  but  if  the  concrete  is  made 
fairly  compact,  and  especially  if  its  surface  is  water-proofed,  the 
amount  of  lime  removed  may  usually  be  neglected.  In  many 
cases  the  removal  of  the  lime  may  be  prevented  by  adding 
trass  in  place  of  part  of  the  sand  (pp.  159,  205). 

Sea  water  appears  to  have  a  peculiarly  destructive  action 
on  some  kinds  of  concrete,  and  its  behaviour  is,  therefore,  of 
great  importance  to  those  engaged  in  the  construction  of 
harbours,  docks,  breakwaters  and  other  maritime  work. 

The  action  of  sea  water  is  partly  physical,  and  the  impact 
force  of  the  waves  and  the  grinding  action  of  sand,  shingle  and 
pebbles,  are  not  infrequently  of  greater  importance  than  the 
purely    chemical    corrosion.     Against    much    of    this    purely 
physical  action  man  is  powerless  ;    he  can  only  reduce  it  by 
the   use    of   hard    aggregates    and   of   dense,    well-compacted 
concrete  sufficiently  rich  in  cement  to  resist  for  a  reasonable 
number   of   years.     That   such   remarkably   resistant   marine 
works  can  be  constructed  is,  in  fact,  one  of  the  wonders  of 
modern  engineering.     Strength  and  density  are  the  two  chief 
properties   on   which   modern   engineers  rely  in  constructing 
concrete  which  will  withstand  the  physical  action  of  the  waves. 
Experiments  on  a  large  scale  in  Germany  and  Copenhagen 
have  shown  that  the  succession  of  saturation,  drying,  freezing, 
etc.,  to  which  marine  work  is  subjected  by  the  changes  in  the 
water  level  due  to  the  tides,  also  has  a  most  important  effect 
in  causing  the  destruction  of  the  masonry.     The  action  of 
frost  is  particularly  powerful,  and  the  utter  impossibility  of 
protecting  embankments   and   other  marine   works   from   its 
action  makes  this  all  the  more  serious.     The  general  action  of 
frost  on  concrete  structures  is  described  on  p.  271. 

A  further  factor  which  has  not  received  the  attention  it 
deserves  is  the  percolation  of  salt  water  into  the  interior  of 
the  concrete,  and  the  crystallisation  of  the  salt  as  the  water 
evaporates.  The  effect  of  this  crystallising  in  cement  blocks, 
which  are  only  immersed  at  long  intervals,  is  similar  to  that 
of  frost. 


EFFECT  OF  SEA  WATER  265 

The  chemical  action  of  sea  water  is  due  to  that  of  a  variety 
of  substances,  of  which  carbon,  sulphur  and  magnesia 
compounds  are  usually  considered  to  be  the  most  important. 

Sea  water  in  the  neighbourhood  of  land  usually  contains  a 
notable  amount  of  carbon  dioxide,  and  this  dissolves  out  a 
portion  of  the  lime  in  the  concrete,  leaving  the  mass  more  open 
and  accessible.  Generally  speaking,  however,  the  corrosive 
action  of  sea  water  is  confined  to  the  material  quite  close  to 
the  surface  of  the  concrete,  as  a  re-deposition  of  some  of  the 
dissolved  matter  not  infrequently  occurs  in  the  pores  of  the 
concrete.  Excessive  action  in  a  compact  concrete  is  also 
prevented  by  the  slowness  with  which  the  water  can  enter 
the  material. 

The  constituent  of  sea  water,  which  is  generally  regarded  as 
being  the  most  detrimental  to  concrete,  is  magnesia  in  the  form 
of  magnesium  sulphate,  or  chloride,  as  these  salts  decompose 
cement,  forming  calcium  sulphate  and  chloride.  The  calcium 
sulphate  may  then  form  crystalline  calcium  aluminium  sulphate 
with  considerable  increase  in  volume,  and  thereby  tend  to 
destroy  the  concrete.  Hence,  the  effect  of  sea  water  is  greatest 
when  the  concrete  is  porous,  and  is  at  a  minimum  when  the 
concrete  is  impervious. 

C.  von  Blaese  has  found  that  half  the  lime  in  a  cement  is 
removed  as  calcium  chloride  when  the  cement  is  treated  with 
an  excess  of  a  6  per  cent,  solution  of  magnesium  chloride. 
So  intense  a  reaction  is  disputed  by  Kallauner,  though  the 
latter  agrees  that  all  magnesium  salts  decompose  cement,  the 
chief  reaction  occurring  between  them  and  the  calcium  hydrate 
formed  during  the  setting,  colloidal  magnesium  hydroxide 
and  a  calcium  salt  being  produced.  In  contradiction  to  von 
Blaese,  Kallauner  also  observed  that  where  the  calcium  salt 
is  almost  insoluble  in  water  the  volume  of  the  cement  was 
altered  and  cracks  formed  ;  when  soluble  calcium  salts  are 
produced  the  volume  of  the  cement  is  not  affected,  but  its 
strength  is  greatly  reduced. 

In  other  words,  it  is  not  the  small  proportion  of  sulphate  in 
the  original  cement  which  is  important  in  submarine  work, 
but  the  permeability  of  the  concrete  which  permits  the 
magnesium  sulphate  in  the  sea  water  to  react  with  the  cement, 


266          SPECIAL  PROPERTIES  OF  CONCRETE 

If  this  is  the  true  explanation  of  the  destructive  action 
of  sea  water,  the  latter  may  be  overcome  by  the  use  of 
an  impervious  concrete,  prepared  from  a  more  finely-ground 
cement  and  a  more  carefully  graded  and  proportioned 
concrete. 

The  precise  reactions  produced  by  magnesia  compounds  have 
not,  however,  been  studied  with  great  accuracy,  and  there  is 
ample  room  for  a  further  investigation  of  the  subject.  More- 
over, the  calcium  sulphate  in  sea  water  appears  to  have  a 
direct  chemical  action  on  cement,  according  to  E.  Candlot  a 
calcium  sulpho-aluminate  being  formed.  Cements  which 
contain  little  or  no  alumina  (such  as  Teil  hydraulic  lime  or 
grappier  cement)  appear  to  be  less  affected  by  calcium  and 
magnesium  sulphates,  and  the  conclusion  has  been  drawn  that 
the  action  of  these  sulphates  must  be  largely  confined  to  the 
alumina  in  the  cement.  For  this  reason  iron  cements,  which 
are  free  from  alumina,  are  sometimes  used  for  maritime 
work.  W.  and  D.  Asch,  on  the  contrary,  maintain  that  the 
action  of  sea  water  depends,  to  a  larger  extent  than  is 
generally  supposed,  on  the  constitution  of  the  cement, 
and  that  highly  aluminous  cements  are  quite  resistant  to 
magnesium  and  calcium  sulphates,  providing  that  such 
cements  possess  the  correct  constitution.  In  accordance 
with  the  theory  of  these  investigators  (p.  55)  most 
Portland  cements  contain  hydroxyl  groups  attached  to  the 
aluminum  rings,  these  being  combined  with  lime  or  other 
base,  thus  :— 

5CaO  KG  5CaO 

-ICaO 


Si 


Al 


Si 


/ 
5CaO  KG  5CaO 


In  such  a  compound  the  lime  attached  to  the  aluminium 
ring  behaves  differently  from  that  attached  to  the  silicon 
rings.  Thus,  on  treating  such  a  cement  with  water,  part  of 
the  lime  is  hydrolysed  and  separated  as  calcium  hydrate,  thus 


EFFECT  OF  SEA  WATER 


267 


The  compound  formed  is,  according  to  Asch's  theory,  of  the 
type 

HO.Ca.O  O.Ca.OH 


HO' 

Ca'.O 

O.Ca 

'.OH 

OK 

' 

HO.Ca 

.o\ 

x\/\/\/0. 

Ca.OH 

HO.Ca 

.o/ 

\0. 

Ca.OH 

HO.Ca 

•  0\ 

/o. 

Ca.OH 

HO.Ca 

.o/ 

\/\/\/\0. 

Ca.OH 

HO. 

Ca  .  01 

OK 

^0  .Ca.  OH 

HO 

.Ca. 

0 

O  .Ca 

.OH 

The  two  OK-groups  attached  to  the  aluminium  ring  have  a 
strong  tendency  to  react  with  groups  of  acid  radicals,  such  as 
-  S02OH,  —SO'OH,  etc.,  so  that  the  destructive  action  of  the 
sulphates  in  sea  water  is  a  necessary  consequence  of  Asch's 
theory.  From  this  it  follows  that  if  hydraulic  cements  can 
be  prepared  which  cannot  contain  these  two  hydroxyl  groups, 
they  would  be  resistant  to  sulphates.  Such  cements  would 
probably  be  of  the  following  types  :— 


Si    Al 


TYPE  A. 


TYPE  B. 


TYPE  C. 


(See  p.  56)  W.  and  D.  Asch's  conclusions  are  too  new,  as  yet 
to  be  generally  accepted.  In  this  connection  it  is  interesting 
to  note  that  Schuljatschenko,  J.  A.  van  der  Kloes  and  the 
author  have  each  independently  found  that  the  replacement 
of  some  of  the  sand  in  the  cement  by  trass  improves  concrete 
for  maritime  work. 

The  difficulty  of  ascertaining  accurately  the  action  of  acid 
radicals  (including  sulphates,  etc.,  in  sea  water)  on  cement  is 
greatly  increased  by  the  relative  impermeability  of  the  concrete, 


268          SPECIAL  PROPERTIES  OF  CONCRETE 

This  has  the  effect  of  reducing  the  apparent  action  of  the 
solution,  and  has  led  to  the  conclusion  that  sea  water  per  se 
has  no  action  on  concrete.  Thus,  the  results  of  the  large  scale 
tests  carried  out  by  the  Scandinavian  Portland  cement  manu- 
facturers are  regarded  as  showing  that,  provided  a  good  quality 
of  Portland  cement  is  used  and  that  the  mortar  is  rich  (1  of 
cement  to  2  of  sand)  and  compact,  the  chemical  action  of 
sea  water  will  be  inappreciable  in  ten  years.  A  loose  mortar 
or  one  made  with  hydraulic  lime,  on  the  contrary,  is  soon 
disintegrated,  except  in  unusually  mild  climates,  such  as  the 
Mediterranean. 

In  order  to  withstand  the  severe  physical  conditions,  concrete 
blocks  for  marine  work  should  be  allowed  to  harden  in  air  for 
as  long  as  possible  before  being  immersed  in  the  sea.  The 
results  of  some  important  German  tests  show  that  a  hardening 
period  of  a  year  is  desirable.  A  still  more  recent  report  of 
experiments  by  R.  L.  Humphrey  states  that  "  while  opinions 
differ  as  to  its  durability  in  fresh  and  sea  water,  concrete 
mixed  and  placed  so  that  the  resulting  mass  is  of  maximum 
density  affords  ample  resistance  to  the  action  of  both  fresh 
and  sea  water,  especially  if  allowed  to  harden  before  exposure." 

The  consensus  of  opinion  on  this  point  seems  to  be  as  follows  : 

(a)  "  Little,  if  any,  damage  is  done  to  even  the  poorest 
concrete  structure  below  water. 

(b)  "  The  principal  destructive  actions  occur  between  tides, 
generally  at  the  mean-tide  line. 

(c)  "  Where  the  concrete  has  disintegrated  in  sea  water  this 
is  undoubtedly  caused  by  freezing,   for  the  reason  that  in 
tropical  waters,  where  there  is  no  freezing,  the  structures  do 
not  show  this  deterioration,  excepting  in  cases  where  sea  water 
has  been  used  in  the  mixing  and  the  concrete  has  been  placed 
through  sea  water,  thereby  producing  an  inferior  structure 
unsuited  to  resist  sea  water  action. 

(d)  "  While  there  may  be  some  chemical  action  after  the 
concrete  is  weakened  through  freezing,  it  is  a  fact  that  there 
is  little  or  no  chemical  action  on  a  properly  proportioned, 
mixed,  and  placed  concrete. 

(e)  "  It  seems  to  be  a  fact  that  it  is  desirable  that  the  cement 
used  in  making  concrete  to  be  exposed  to  sea  water  shall 


EFFECT  OF  SEA  WATER  269 

contain  sufficient  hydraulic  materials  as  will  satisfy  whatever 
excess  lime  there  may  be  in  the  hardened  cement.  This  is 
accomplished  in  many  parts  of  Europe  through  the  addition  of 
trass  or  pozzolana  to  hydraulic  cement.  The  trass  combines 
with  the  lime  set  free  from  the  cement,  and  so  increases  its 
strength  and  renders  it  stable  in  sea  water. 

"  It  is  generally  understood  that  the  concrete  must  be  rich  in 
slow-setting  cement  and  must  not  contain  less  cement  than  the 
proportions  :— 

(a)  2  cement,  1  trass,  6  (sand  and  aggregate)  ; 
(/>)  2  cement,  3  (sand  and  aggregate). 

(/)  "It  is  quite  apparent  that  one  of  the  prime  essentials 
for  a  concrete  structure  that  will  be  immune  against  sea  water 
action  is  that  the  surface  shall  be  dense  and  impervious.  An 
attempt  has  been  made  to  secure  this  condition  by  applying 
mortar  under  an  air  pressure  of  upwards  of  SOlbs.,  and  while 
this  undoubtedly  increases  the  density  as  compared  to  hand 
methods,  nevertheless  it  remains  to  be  seen  whether  it  achieves 
the  object  desired. 

(g)  "  Good  practice  demands  that  the  concrete  shall  be  mixed 
a  sufficient  length  of  time,  without  too  much  water,  so  that 
there  results  a  mass  of  viscous  consistency  which  will  flow 
readily  and  yet  the  ingredients  not  separate.  If  such  a 
concrete  is  deposited  under  conditions  which  will  prevent  the 
sea  water  permeating  it  before  it  has  set,  such  concrete  affords 
excellent  resistance  to  sea  water.  Another  method  proposed 
has  been  to  deposit  the  concrete  in  tremie,  as  with  this  form 
of  construction  only  the  upper  surface  comes  in  contact  with 
the  water,  and  there  results  a  concrete  which  is  not  affected 
by  sea  water  action.  (See  p.  192.) 

(h)  "  When  reinforced  concrete  is  used  in  sea  water  it  is 
essential  that  the  aggregate  shall  be  a  hard,  dense  material 
of  low  absorption,  and  that  the  reinforcement  be  protected 
by  a  coating  of  at  least  one  inch  of  trass-like  mortar." 

The  corrosion  of  reinforced  concrete  is  much  more  serious 
than  that  of  plain  mass  concrete,  as  once  the  corrosive  agent 
attacks  the  steel  reinforcement  the  process  of  rusting  will 
continue  at  an  increased  rate.  As  previously  explained, 


270          SPECIAL  PROPERTIES  OF  CONCRETE 

steel  and  iron  are  ordinarily  protected  from  rusting  by  a  coating 
of  concrete,  but  this  does  not  apply  to  an  exceptionally  open 
and  porous  material  such  as  is  produced  by  corrosion  on  the 
concrete  itself.  It  is  also  a  fact  that  if  concrete  is  mixed 
with  sea  water,  or  with  sea  sand,  or  if  it  has  salt  mixed  with  it, 
and  it  is  subsequently  exposed  to  dampness,  the  reinforcement 
will  corrode.  This  was  shown  by  examples  on  a  rather  extended 
scale  some  years  ago  when  J.  S.  Sewell  had  two  experimental 
slabs  of  reinforced  concrete  made  of  identical  composition, 
except  that  one  was  mixed  with  sea  water  and  one  with  fresh 
water.  They  were  exposed  to  the  weather  on  a  roof  in 
Washington,  D.C.,  for  some  months.  At  the  end  of  that  time 
the  reinforcement  in  the  sea  water  slab  was  badly  corroded, 
while  that  in  the  other  was  entirely  untouched.  It  is  of 
importance,  therefore,  that  the  ingredients  used  in  mixing 
concrete  for  hydraulic  works  should  contain  no  corrosive 
material  in  themselves  if  the  concrete  is  to  be  reinforced. 

The  electrolytic  corrosion  of  the  steel  reinforcement  in  concrete 
is  a  matter  requiring  serious  attention.  The  external  symptoms 
usually  take  the  form  of  deep  cracks  in  the  concrete,  usually 
on  the  under  side  of  beams  or  vertically  in  columns  and 
extending  to  the  reinforcement.  Wherever  the  steel  is  visible 
it  will  be  found  to  be  rusty.  Sometimes  the  damage  is  so 
great  that  larger  flakes  of  concrete  fall  away,  exposing  the 
reinforcement.  Leakage  from  electric  lighting  or  tramway 
circuits  is  the  usual  cause.  H.  P.  Brown  has  found  that 
rubber  covered  wires  in  japanned  steel  conduits  are  not 
satisfactory  in  preventing  leakage.  He  also  recommends 
that  the  intermediate  wire  in  three-wire  systems  and  the 
secondary  circuit  of  a  transformer  should  not  be  earthed  within 
a  concrete  building.  All  the  reinforcing  steel  in  a  structure 
should  be  in  electrical  connection  by  means  of  binding  wire. 
Gas,  water  and  steam  pipes  should  be  electrically  insulated 
where  they  pass  through  concrete  walls,  etc.  In  new  buildings 
of  steel  or  reinforced  concrete  the  foundation  steel  should  rest 
on  good  concrete. 

Providing,  however,  that  the  concrete  is  sufficiently  compact 
and  that  acids  and  other  corrosive  agents  are  kept  away 
from  it,  there  is  no  likelihood  of  ordinary,  well-made  concrete 


EFFECT  OF  FROST  271 

being  sufficiently  porous  to  permit  the  steel  reinforcement 
to  become  rusty. 

Frost. — The  resistance  of  hardened  concrete  to  frost  depends 
largely  on  the  density  or  compactness  of  the  concrete.  If  the 
material  is  so  impervious  that  no  appreciable  amount  of  water 
can  penetrate  it,  the  action  of  frost  will  be  infinitesimal.  If, 
on  the  contrary,  the  permeability  of  the  concrete  is  great, 
water  will  penetrate  easily  and,  on  freezing,  will  expand  and 
so  break  up  the  bond  between  the  particles  forming  the  concrete. 
Each  repetition  of  the  frost  increases  the  effect  until  finally, 
if  the  frosts  are  sufficiently  numerous  and  severe,  the  concrete 
will  be  completely  disintegrated. 

As  well-made  concrete  is  not  particularly  permeable,  the 
action  of  frost  is  unimportant  except  where  the  concrete  is 
used  in  harbours,  piers,  etc.,  and  in  other  exceptionally  exposed 
positions.  What  is  required  is  a  well-proportioned  mixture 
to  which  a  liberal,  though  not  excessive,  amount  of  water  has 
been  added,  the  concrete  being  carefully  tamped  and  protected 
from  anything  which  will  wash  out  any  ingredients  before  the 
cement  is  properly  hardened.  Such  a  mixture,  especially  if  it 
contains  trass,  has  been  found  in  innumerable  instances  to 
fulfil  all  requirements  respecting  resistance  to  frost. 

The  durability  of  concrete  has  long  been  established  beyond 
all  doubt,  for  in  spite  of  the  fact  that  early  Roman  and  other 
buildings  were  made  from  materials  obviously  inferior  to  modern 
Portland  cement,  they  still  exist  after  2,000  or  more  years  of 
exposure.  With  the  elaborate  precautions  now  taken  and 
the  superior  cement  now  available,  it  should  not  be  difficult 
to  ensure  modern  and  equally  durable  structures  being 
erected  whenever  they  are  considered  necessary. 

The  fact  that  the  strength  of  concrete  is  increased  by  age 
is  a  further  proof  of  its  durability.  The  rapid  increase  in 
strength  is  indicated  by  the  steepness  of  the  curve  in  Fig.  92. 
In  this  diagram  the  area  enclosed  by  the  dotted  lines  includes 
the  results  of  a  large  number  of  tests  by  Johnson,  whilst  the 
continuous  curve  indicates  the  average  of  these  results.  It 
will  be  observed  that  the  strength  is  expressed  as  a  ratio  of 
tensile  strength  :  compressive  strength. 

In  the  case  of  reinforced  concrete,  the  durability  is  conditioned 


272 


SPECIAL  PROPERTIES  OF  CONCRETE 


by  the  completeness  with  which  the  reinforcements  is  covered. 
If  it  is  exposed  it  will  soon  effect  the  destruction  of  the  whole 
edifice,  but  if  properly  embedded  in  sound  concrete  it  will  last 
indefinitely.  The  only  danger  that  threatens  it  thereafter  is 
the  danger  of  cracks,  which  will  destroy  the  integrity  of  the 
concrete  and  open  up  a  way  for  atmospheric  moisture  or 
water  to  gain  direct  access  to  the  reinforcement.  Such  cracks 
might  be  due  to  shrinkage  in  setting,  to  expansion  and  con- 
traction under  changes  of  temperature,  or  to  deformation 
under  stress.  If  the  components  of  the  concrete  are  well 


fi      9 


f- 


0  24-  6  8  10 

FIG.  92. — Eatio  of  Strength  to  Age.     (Johnson. 


12 

months. 


mixed  in  the  right  proportions  and  kept  wet  while  setting 
there  is  small  danger  of  shrinkage  cracks,  and  they  are  rarely 
seen  in  practice.  Light  reinforcement  is  sometimes  used  with 
a  view  to  preventing  them,  but  it  is  not  easy  to  see  how  it- 
would  be  effective,  for  the  shrinkage  of  the  concrete  would  set 
up  compressive  stresses  in  the  reinforcement,  and  it  is  not 
usually  heavy  enough  to  resist  them  effectively.  The  danger 
of  such  cracks  is  more  imaginary  than  real,  and  an  abundant 
supply  of  water  during  the  mixing,  and  whilst  the  concrete 


CRACKS  IN  CONCRETE  273 

is  hardening  together,  with  protection  from  evaporation  during 
setting,  will  ensure  a  successful  result.  The  reinforcement  will 
generally  be  heavy  enough  to  prevent  shrinkage  cracks,  even 
in  exceptional  cases. 

Cracks  due  to  expansion  and  contraction  after  setting  are 
brought  about  probably  by  a  slight  slipping  of  the  mass  on 
its  bed  during  expansion,  and  by  the  excess  of  frictional 
resistance  over  the  tensile  strength  of  the  concrete  during  the 
subsequent  contraction.  This  trouble  can  be  overcome  by 
proper  reinforcement,  but  it  is  better  to  divide  a  long  wall  or 
other  structure  into  sections,  so  that  each  can  act  as  a  unit. 

Cracks  due  to  deformation  under  stress  only  occur  when  the 
reinforcement  is  stressed  so  that  the  strain  exceeds  the  limit 
of  extensibility  of  the  concrete.  This  can  be  avoided  by 
proper  design  and  workmanship.  This  danger  makes  it  inad- 
visable to  utilise  the  high  working  stresses,  otherwise  permissible 
in  high  carbon  steel,  since  the  modulus  of  elasticity  is  no 
greater  than  with  low  carbon  or  medium  steel.  If  working 
stresses  are  kept  well  within  the  limits  allowable  for  mild 
steel  there. is  no  danger  of  cracks  in  the  concrete. 

With  regard  to  the  tendency  to  crack,  on  account  of  irregular 
settlement  in  the  foundations,  concrete  has  a  great  advantage 
over  other  materials,  as  the  whole  structure  may  be  made 
monolithic,  and  so  could  take  tension  as  well  as  compression. 
Any  tendency  to  cant  sets  up  tension  in  some  member,  which 
in  ordinary  construction  makes  itself  evident  by  cracks  and 
fissures. 

Efflorescence  or  "  scum  "  on  concrete  is  due  to  the  presence 
of  soluble  salts  in  the  aggregate,  sand  or  water,  and,  much  less 
frequently,  in  the  cement.  These  soluble  salts  are  carried  to 
the  surface  of  the  concrete  during  the  drying  and  are  deposited 
there  as  the  water  evaporates.  Occasionally,  efflorescence  in 
concrete  is  caused  by  water  containing  soluble  salts,  being 
drawn  up  from  the  foundations  by  capillary  attraction. 

The  only  means  of  preventing  efflorescence  consists  in  using 
materials  as  free  as  possible  from  soluble  salts. 

The  discoloration  of  concrete  is  usually  due  to  oil  or  to  dirt 
on  the  forms,  but  in  some  cases  it  is  caused  by  impurities  in 
the  aggregate,  which  rise  to  the  surface  and  form  stains. 

c.  T 


274-          SPECIAL  PROPERTIES  OF  CONCRETE 

Failures  in  concrete  structures  have  been  both  numerous 
and  serious,  so  that  a  few  words  with  reference  to  them  may 
be  included  here.  The  causes  of  failures  may  be  divided  into 
two  groups — unpreventable  and  preventable. 

Failures  from  unavoidable  causes  include  earthquakes, 
inundation,  lightning,  tempest,  fire,  explosions  and  exceptional 
shocks.  As  they  are,  by  their  nature,  unavoidable  nothing 
further  need  be  said  about  them  beyond  the  remark  that 
concrete  structures,  if  designed  for  the  purpose,  will  withstand 
most  of  these  forces  as  well  as  or  better  than  any  other  building 
material. 

Failures  from  preventable  causes  are  almost  invariably  due 
to  mistakes  or  carelessness  in  construction.  If,  for  instance, 
the  shuttering  or  forms  are  too  weak,  they  may  collapse  and 
bring  about  the  partial  destruction  of  the  building.  Badly 
arranged  pillars  have  been  a  frequent  cause  of  collapse,  and 
mistakes  in  the  erection  of  the  building  have  been  even  more 
serious.  What,  for  instance,  can  be  thought  of  the  contractor 
who  built  a  bridge  of  reinforced  concrete  and  omitted  to  put 
the  stirrups  on  the  reinforcement  ?  Other  mistakes  not 
infrequently  observed — particularly  when  the  work  is  under 
the  charge  of  men  who  are  not  skilled  in  concrete  construction 
— or  the  use  of  dirty  water,  unsuitable  aggregate,  unwashed 
or  badly  washed  sand  or  low  grade  cement.  If  the  concrete 
is  placed  improperly,  and  particularly  when  the  adhesion 
between  two  successive  layers  is  imperfect  (p.  190),  failures 
are  almost  sure  to  result. 

Some  of  the  common  causes  of  failure  in  the  making  of 
concrete  are  as  follows  : — 

(a)  Too  much  water  during  mixing,  or  water  carelessly 
applied,  or  an  insufficient  quantity  of  water. 

(6)  An  insufficiently  graded  aggregate,  particularly  one 
containing  only  very  coarse  material,  or  one  with  too  much 
sand  or  loam. 

(c)  Incomplete   incorporation    of    the    aggregate    with   the 
cement. 

(d)  Allowing  the  concrete  to  stand  until  the  setting  action 
has  commenced  and  then  regauging  before  use,  or  using  up 
old  concrete. 


CAUSES  OF  FAILURE  275 

(e)  Bad  cement.  A  branded  British  cement  is  usually 
reliable,  but  it  must  not  be  too  quick  setting  or  lumpy  or  caked. 

(/)  Unsuitable  aggregates.  It  is  particularly  necessary  to 
avoid  using  any  aggregate  that  may  be  handy.  The  best  for 
the  purpose  should  be  chosen.  It  is  also  unwise  to  accept 
aggregates  on  the  basis  of  small  samples.  Natural  aggregates 
are  risky  on  account  of  the  variations  in  the  proportion  of  sand 
they  contain,  and  should  be  screened  before  use  (p.  156). 

(g)  Rendering  cement  work  on  dry  foundations  and  without 
thoroughly  saturating  the  surface  with  water. 

(h)  Dirty  aggregate  or  water  containing  earthy  matter,  clay, 
loam,  or  strongly  coloured  water. 

(«)  Carelessness  in  proportioning  mixtures. 

(j)  Excessive  ramming  or  tamping. 

(k)  Weak  centering,  sparse  timbering  and  badly  arranged 
forms. 

(I)  Premature  removal  of  forms. 

(m)  Excessive  trowelling  or  floating  of  cement  surfaces. 

(n)  Erroneous  design  or  arrangement  of  the  reinforcement. 

Failures  due  to  overlooking  springs  or  subsoil  water,  to  errors 
in  calculating  the  strength  and  arrangement  of  the  various 
portions  of  the  structure,  or  to  making  insufficient  allowance 
for  the  face  of  the  sea  or  wind,  are  by  no  means  unknown, 
and  the  danger  of  failure  by  electric  currents  has  already 
been  mentioned  (p.  270). 

Ignorance  and  hurry  are,  in  fact,  the  two  great  causes  of 
concrete  failures.  There  is  frequently  an  undue  stress  put 
upon  the  contractors  or  on  the  workman  to  use  as  little  shutter- 
ing timber  as  possible,  and  to  remove  the  forms  too  soon. 
It  must  be  remembered  that  concrete  does  not  gain  its  total 
strength  immediately,  but  requires  several  days  before  it  is 
strong  enough  to  enable  the  supports  to  be  removed  with 
safety.  Shortage  of  timber  and  undue  haste  in  construction 
are  therefore  extremely  serious,  especially  if  the  responsible 
persons  are  not  too  well  informed  as  to  the  properties  of 
concrete. 

It  is  a  noteworthy  fact  that  practically  no  "  mysterious  " 
failures  have  ever  occurred  in  concrete  structures  long  after 
they  have  been  completed,  so  that  it  may  be  assumed  that  in 

T  2 


276          SPECIAL  PKOPERTIES  OF  CONCRETE 

almost  every  case  the  failure  has  been  due  to  some  error  in 
construction.  The  carelessness  and  ignorance  of  some  con- 
tractors who  undertake  concrete  work,  and  the  indifference 
they  show  with  regard  to  inspection  during  construction,  make 
it  essential  that  all  concrete  work  should  be  placed  in  the  hands 
of  skilled  and  trustworthy  men.  This  is  confirmed  by  the 
fact  that  all  localities  in  which  a  good  code  of  regulations  is 
enforced  have  proved  free  from  great  disasters  of  a  preventable 
nature. 

The  conclusion  seems  justified  that  the  objections  which 
have  been  urged  against  the  use  of  concrete  as  a  structural 
material  are  either  imaginary  or  can  be  overcome  by  practicable 
methods,  especially  as  many  of  them  arose  when  the  subject 
was  not  so  well  understood  as  at  present.  That  this  conclusion 
is  justified  is  abundantly  proved  by  the  increasing  and  successful 
use  of  the  material  in  permanent  structures  of  the  most  varied 
kinds  in  all  parts  of  the  world.  That  concrete  is  not  suitable 
for  every  kind  of  imaginable  purpose  is  not  surprising,  but  its 
uses  are  so  multifarious  that  there  is  no  need  to  attempt  to 
employ  it  for  purposes  or  in  situations  for  which  it  is  not 
eminently  suitable. 


CHAPTER   X 

TESTING    CONCRETE 

THE  greater  part  of  the  testing  applied  to  concrete  is  in 
relation  to  the  raw  materials  of  which  it  is  made. 

Cement. — The  tests  generally  used  for  Portland  cement 
have  been  described  in  Chapter  V. 

Aggregates. — These  are  seldom  subjected  to  special  tests, 
though  it  is  very  desirable  that  they  should  be  tested  in  a 
manner  similar  to  cement.  The  proportion  of  aggregate  of 
various  grades  or  sizes  is  usually  specified  by  the  engineer  or 
architect  in  charge  of  the  work,  and  precautions  should  be  taken 
to  ensure  these  proportions  being  maintained. 

The  limits  of  size  of  particles  in  the  various  grades  of 
aggregate  should  also  be  checked,  as  otherwise  very  inferior 
concrete  may  be  produced.  It  is  particularly  desirable  that 
all  sand  should  be  removed  from  the  coarser  grades  of 
aggregate. 

The  specification  of  a  minimum  crushing  strength  for  the 
aggregate  is  seldom  made,  it  being  generally  assumed  that 
if  the  finished  concrete  stands  the  necessary  tests,  the  materials 
of  which  it  is  composed  must  be  ipse  facto  satisfactory.  The 
usual  limitations  are  stated  on  p.  154,  but  far  too  little  atten- 
tion is  paid  to  the  correct  grading  of  the  aggregates. 

Sand. — The  sand  must  be  tested  to  ensure  its  freedom 
from  clay  and  dust,  it  being  now  recognised  that  all  material 
which  will  pass  through  an  aperture  ^  inch  by  ^\y  inch  should 
be  rejected  as  harmful.  Dust  and  extremely  fine  sand  greatly 
reduce  the  strength  of  any  concrete  in  which  they  may  be 
present. 

Steel. — The  tests  imposed  on  the  steel  used  for  reinforced 
concrete  have  been  mentioned  on  p.  211.  It  is  necessary 
to  check  the  sizes  carefully,  as  any  reduction  in  the  cross-section 


278 


TESTING  CONCRETE 


may  be  very  serious.  Every  piece  of  steel  used  should  also 
be  carefully  examined  for  flaws  and  irregularities.  Steel 
which  shows  a  granular  fracture  should  be  avoided. 

The  crushing  strength  of  concrete  is  the  chief  test  imposed. 
The  test  pieces  are  cubes  with  either  3-inch,  4-inch  or  6-inch 
sides,  smaller  ones  being  unreliable.  The  test  pieces  are  made 
on  the  works  at  the  same  time  as  the  various  batches  of  concrete 
are  made.  The  test-cubes  should  be  kept  slightly  damped 


FIG.  93. — Machine  for  Crushing  Tests. 

for  seven  days.  For  tests  to  be  made  after  longer  periods, 
the  cubes  should  be  kept  under  cover  so  as  to  protect  them 
from  dust,  rain  and  direct  sunlight. 

Care  is  needed  to  keep  the  amount  of  tamping  as  similar 
as  possible  to  that  employed  on  the  larger  masses  of  concrete. 

Sometimes  blocks  of  concrete  are  cut  from  work  actually 


CRUSHING  STRENGTH  279 

under  construction,  but  this  very  greatly  increases  the  cost 
of  the  test  on  account  of  the  difficulty  of  cutting  the  sample. 

The  Joint  Committee  under  the  auspices  of  the  Royal 
Institute  of  British  Architects  has  recommended  that  the 
cubes  should  be  made  "  before  the  detailed  designs  for  an 
important  piece  of  work  are  prepared,"  and  that  the  tests 
should  be  made  twenty-eight  days  after  moulding.  At  least 
six  cubes  should  be  used  in  each  test.  "  In  the  case  of  concrete 
made  in  proportions  of  1  cement,  2  sand,  4  hard  stone,  the 
strength  should  not  be  less  than  1,800  Ibs.  per  square  inch." 
Such  a  concrete  should  develop  a  strength  of  at  least  2,400  Ibs. 
per  square  inch  after  ninety  days. 

Other  authorities  recommend  tests  to  be  made  at  the  end  of 
seven,  twenty-eight,  fifty-six,  ninety  and  365  days,  and  some 
at  the  end  of  two,  three,  four,  or  five  years.  Owing  to  the 
speed  with  which  large  concrete  buildings  are  erected  a  seven- 
days'  test  is  desirable,  and  though  it  is  claimed  that  its  results 
are  unreliable,  and  that  at  least  twenty-eight  days  should 
elapse  before  testing,  there  seems  much  probability  of  a 
short-time  test  being  brought  into  regular  use. 

Tests  on  cubes  or  other  small  samples  are  seldom  very 
reliable  ;  tests  on  full-sized  columns,  beams  or  blocks  are  much 
more  accurate  and  preferable,  but  they  require  such  special 
and  powerful  machinery,  as  to  necessitate  the  samples  being 
sent  to  a  testing  station  fitted  for  the  purpose. 

In  testing  columns  of  reinforced  concrete  for  crushing  strength 
it  will  usually  be  found  that  they  break  at  the  end  which  was 
uppermost  during  manufacture,  thus  showing  that  the  solidity 
of  a  concrete  column  diminishes  towards  the  upper  end.  This 
is  confirmed  by  tests  of  the  specific  gravity  of  the  various  parts 
of  the  columns,  the  upper  end  having  a  much  lower  density 
than  the  lower  one. 

In  testing  floors  and  slabs  care  must  be  taken  to  distribute 
the  load  uniformly.  Bars  of  pig  iron  and  building  bricks  both 
tend  to  form  arches  and  cause  an  irregular  distribution  of 
the  load. 

Loading  Tests. — In  view  of  the  general  unsatisfactoriness  of 
testing  cubes  of  concrete,  F.  von  Emperger  proposed,  in  1903, 
that  they  should  be  replaced  by  loading  tests  on  specially 


280 


TESTING  CONCRETE 


prepared  reinforced  concrete  beams.  The  Emperger  test  is 
intended  to  be  carried  out  on  the  spot,  the  test  beams  being 
prepared  from  the  materials  there  in  use,  and  the  testing 
apparatus  being  fitted  up  close  at  hand  from  parts  which  are 
packed  in  portable  cases,  which  also  contain  bent  steel  rods 
as  models  to  be  copied  by  the  workmen  preparing  the  test 
beams.  It  is  best  to  prepare  four  such  beams  for  each  test, 
two  of  which  are  reinforced  with  a  single  longitudinal  rod 
(Type  I.),  and  two  with  two  parallel  rods  (Type  II.).  The 


FIG.  94. — Chart  for  use  with  Emperger's  Test. 

beams  are  2-30  metres  (6  feet  8  inches)  long,  7  cm.  (2f  inches) 
broad,  and  10  cm.  (4  inches)  deep.  The  reinforcing  rods  are 
12  mm.  (|  inch)  in  diameter,  and  are  bent  up  and  turned  over 
at  the  ends,  as  shown  at  the  top  of  Fig.  94.  The  maximum 
stress  obta  nable  in  concrete  by  the  use  of  this  type  of  beam  is 
considerably  in  excess  of  that  ever  attained  in  actual  practice. 
The  error  due  to  accidental  shifting  of  the  reinforcing  rods 
during  the  preparation  of  the  beams  does  not  exceed  2  per 
cent,  of  the  stress.  The  beams  are  so  chosen  that  the  breaking 

p 
moment  (M0)  is  equal  to  half  the  breaking  load,  i.e..  M0  =  — . 


EMPERGERVS  LOADING  TEST 


281 


The  moulds  are  double,  made  of  wood,  lined  with  sheet  iron. 
They  are  oiled  and  fitted  together  by  means  of  distance  pieces  ; 
two  loops  to  serve  as  handles  are  placed  in  position,  and  the 
sides  are  accurately  adjusted  by  means  of  a  gauge  until  the 
middle  third  has  exactly  the  correct  width.  The  reinforcing 
rods  are  then  laid  in  position  on  the  distance  irons  and  handles. 
The  concrete  is  introduced,  and  tamped  in  the  same  way  as 
in  the  construction  of  a  floor.  The  moulds  may  be  removed 
in  two  or  three  days,  after  which  care  is  taken  that  the  beams 
are  exposed  to  the  same  conditions  of  temperature  and  weather 


136 


FIG.  95. — Emperger's  Loading  Test. 


as  the  work  to  be  controlled.  If  four  beams  are  used,  two  are 
tested  after  three  weeks,  and  two  after  six  weeks,  but  for  rapid 
tests  intended  to  determine  whether  centering  may  be  removed, 
a  single  beam  is  sufficient. 

The  framework  used  in  making  the  loading  test  is  shown  in 
Fig.  95.  The  two  hardwood  knife  edges,  with  iron  edges,  are 
placed  exactly  2  metres  apart,  and  the  load  is  then  hung  from 
two  wooden  riders,  placed  as  shown  at  a  distance  apart  of 
50cm.  (1  foot  8  inches).  The  chains  must  allow  the  loading 


282  TESTING  CONCRETE 

platform  to  swing  freely,  without  allowing  too  great  a  fall 
when  destruction  occurs.  It  is  advisable  to  place  a  board 
(not  a  thick  plank)  immediately  under  the  beam  to  receive 
the  broken  halves  and  prevent  complete  collapse.  A  vertical 
scale  may  be  attached  to  this  board  at  the  middle  point  to 
measure  the  deflection  before  fracture.  The  load  is  applied 
by  means  of  bricks,  which  are  added  symmetrically,  according 
to  a  definite  scheme,  by  two  workmen,  and  are  counted  until 
fracture  occurs.  The  breaking  load  is  then  made  up  of  the 
weights  of  the  bricks  and  the  loading  apparatus,  and  two- 
thirds  of  the  weight  of  the  beam.  (When  loading  is 
applied  at  four  points,  corresponding  with  a  test  under 
distributed  load,  the  whole  weight  of  the  beam  must  be 
included.) 

Calling  the  breaking  load  P,  the  compressive  stress  (<r7>) 
reached  in  the  concrete  is,  for  Type  I.  of  reinforcement,  cry>  = 
0-384  P,  and  for  Type  II.,  crB=  0-3285  P.  It  remains  to  be 
determined,  however,  whether  this  stress  was  the  cause  of 
fracture — that  is,  whether  the  maximum  compressive  strength 
was  utilised.  This  is  most  readily  determined  by  inserting 
p 

M0  =   -JT  in  the  graphical   diagram  here    shown.     It   will   be 

Zi 

seen  from  this  that  for  the  best  qualities  of  concrete  it  is 
advisable  to  use  Type  II.,  with  4  per  cent,  of  reinforcement. 

In  the  diagram  (Fig.  94),  the  percentage  of  reinforcement  is 
plotted  as  abscissae,  and  the  values  of  the  breaking  moment, 
M0,  as  ordinates.  The  strength  of  the  steel  is  taken  as  its 
elastic  limit  (which  is  generally  3,500  kgs.  per  square  centi- 
metre or  49,700  Ibs.  per  square  inch)  ;  it  is  shown  as  a  thick 

P 
line  on  the  diagram.     If  the  value  of  M0  =  —  falls  below  the 

•steel  curve,  fracture  is  due  to  failure  of  the  concrete,  and  the 
strength  of  this  may  be  determined  by  the  aid  of  the  second 
group  of  curves  ;  if  above,  fracture  is  due  to  the  steel,  and  it 
cannot  be  known  whether  the  maximum  strength  of  the 
concrete  has  been  reached  or  not.  Actually,  failure  of  the 
steel  is  very  rarely  observed.  In  the  neighbourhood  of  its 
elastic  limit,  the  steel  unloads  itself  at  the  expense  of  the 
concrete,  and  the  resulting  shifting  of  the  neutral  axis  upwards, 


EMPERGER'S  LOADING  TEST  283 

and  increase  of  the  compressive  stress  in  the  concrete,  causes 
fracture  of  the  concrete,  really  from  a  secondary  cause. 

A  comparison  of  Dr.  von  Emperger's  results  with  those 
obtained  by  Professor  Morsch  and  by  Professor  von  Bach, 
shows  that  greater  uniformity  is  found  in  the  bending  tests 
than  in  the  compression  tests  with  cubes. 

The  average  variation  in  the  results  of  the  bending  tests  is 
about  7  per  cent,  to  8  per  cent,  of  the  mean  value,  these  varia- 
tions being  attributable  to  differences  of  temperature,  moistness 
of  air,  etc.,  and  in  some  cases  also  to  the  fracture  of  the  beam 
taking  place  at  different  parts.  The  corresponding  compression 
tests  with  cubes  exhibit  far  greater  irregularities  and  may 
easily  reach  25  per  cent.  It  is  important  that  the  reinforce- 
ment should  be  accurately  placed,  as  quite  small  errors  in  this 
respect  cause  a  serious  loss  of  strength.  Excluding  the  most 
irregular  values,  the  ratio  of  the  breaking  stress  in  bending 
tests  to  that  in  compression  tests  is  from  1-3  to  1-6  :  1. 

The  cheapness  and  simplicity  of  the  test,  as  well  as  its 
trustworthiness,  are  sufficient  reasons  for  its  introduction  as 
a  means  of  systematic  control  of  building  operations  and  for  the 
regular  courses  of  instruction  in  several  important  colleges. 
It  has  been  appreciated  wherever  used,  by  engineers,  con- 
tractors and  workmen  alike. 

Concrete  structures,  particularly  floors,  are  frequently  tested 
by  the  direct  application  of  a  load  one-and-a-half  times  that 
which  they  have  been  designed  to  carry,  the  deflection  being 
noted.  The  great  disadvantage  of  this  method  of  testing 
is  that  the  structure  may  be  strained  and  permanently  injured. 
Hence,  it  is  generally  desirable  only  to  apply  the  designed 
load.  If  no  undue  deflection  then  occurs,  further  loading 
can  serve  no  useful  purpose,  and  may  prove  a  serious  dis- 
advantage. 

The  Joint  Committee  under  the  auspices  of  the  Royal 
Institute  of  British  Architects  emphatically  declare  that  no 
load  tests  on  the  structure  itself  should  be  made  until  at  least 
two  months  after  the  laying  of  the  concrete,  and  that  even  then 
the  test  load  must  not  exceed  one-and-a-half  times  the  acci- 
dental load,  nor  two-thirds  of  the  elastic  limit  of  the  steel 
reinforcement . 


284  TESTING  CONCRETE 

Deflection. — The  maximum  amount  of  deflection  permissible 
can  be  calculated  from  the  bending  moment,  the  elastic 
modulus,  and  the  equivalent  moment  of  inertia,  but  this 
calculation  is  beyond  the  scope  of  the  present  work. 

Permanent  deflection — which  continues  after  the  load  has 
been  removed — is  usually  a  sign  that  the  material  has  been 
overloaded,  though  in  some  instances  it  is  caused  by  a 
"  settling  "  of  the  material  on  the  bedding  or  supports,  and  is 
then  of  minor  significance.  An  extremely  slight  permanent 
deflection  (not  exceeding  J  inch)  is  in  most  cases  unavoidable. 

Experiments  made  by  Van  Ornum  in  1907  show  that 
repeated  applications  of  a  load  increase  the  permanent  deflec- 
tion, but  do  not  alter  the  elastic  deflection.  The  greater  part 
of  the  permanent  set  occurs  on  the  first  application  of  the  load, 
but  each  subsequent  application  increases  it  slightly.  H.  C. 
Berry  and  A.  T.  Goldbeck  have  confirmed  this,  and  find  that 
the  ultimate  strength,  the  maximum  deflection,  and  the 
adhesion  of  the  steel  and  concrete  are  not  materially  affected 
by  one-and-three-quarter  million  repetitions  of  high,  but  safe 
loads. 

Arguments  and  recommendations  based  on  deflections  are 
often  erroneous  unless  made  by  thoroughly  competent  engineers, 
as  the  deflection  varies  according  to  the  distribution  of  the  load, 
the  manner  of  supporting  the  test  piece,  and  the  relation  of 
the  size  of  the  load  to  that  of  the  test  piece. 

Tests  of  the  permeability  of  concrete  are  made  by  measuring 
the  amount  of  water  which  passes  through  a  slab  or  disc  of 
the  concrete  in  a  given  time,  under  a  predetermined  pressure. 
The  slab  or  disc  is  clamped  on  to  the  end  of  a  pipe  into  which 
water  is  pumped  under  pressure.  The  water  passing  through 
the  concrete  is  collected  in  a  measuring  glass.  Unfortunately, 
such  a  method  of  testing  is  far  from  satisfactory,  as  it  necessi- 
tates the  use  of  thin  slabs  and  cannot  be  applied  to  blocks  of 
the  same  thickness  as  the  concrete  structure.  It  is  also  difficult 
to  use  aggregate  of  the  same  coarseness  as  that  used  on  the 
larger  scale. 

More  reliable  results  are  obtained  by  mixing  the  concrete 
and  placing  it  in  a  large  iron  tank  made  strong  enough  to  resist 
the  testing  pressure,  and  tamping  and  finishing  the  surface 


LOADING  TESTS  285 

exactly  as  though  the  iron  tank  were  a  "  form."  Water  is 
supplied  under  pressure,  and  the  water  which  passes  through 
the  concrete  is  collected.  Testing  with  an  excessive  pressure 
is  useless,  as,  if  the  pressure  is  sufficiently  great,  solution  of 
the  hydrated  cement  will  occur,  and  all  concretes  will  be  found 
to  be  permeable.  A  pressure  of  20  Ibs.  per  square  inch  is  the 
maximum  ordinarily  used. 

The  permeability  of  new  concrete  is  not  necessarily  an  indica- 
tion of  bad  workmanship.  Many  tanks  will  exude  a  small 
quantity  of  water  when  first  filled,  but  this  "  weeping  "  ceases 
after  two  or  three  months,  on  account  of  the  re-deposition  of 
some  of  the  constituents  previously  dissolved  by  the  water. 

Where  permeability  is  to  be  tested,  apart  from  pressure,  a 
method  suggested  by  Le  Chatelier  may  be  used.  The  test 
piece  is  immersed  in  a  solution  of  calcium  sulphide,  and  after 
a  suitable  length  of  time  it  is  taken  out,  wiped  dry  and  broken. 
The  iron  compounds  in  the  concrete  will  be  converted  into  iron 
sulphide,  recognisable  as  a  black  or  dark  green  stain,  wherever 
the  solution  has  penetrated.  Well-made  concrete  is  quite 
impermeable  when  tested  in  this  manner. 

Tests  for  concrete  to  be  used  under  sea  water  are  peculiarly 
difficult  and  costly.  It  has  been  shown  repeatedly  that  test 
pieces  immersed  in  still  sea  water  are  useless.  Moreover, 
the  test  pieces  must  be  immersed  for  several  years,  as  the 
changes  occurring  during  the  first  twelve  months  are  irregular, 
and  conclusions  drawn  from  them  are  illusory. 

An  exceedingly  valuable  record  of  numerous  tests  will  be 
found  in  "  Concrete  and  Constructional  Engineering,"  Vols.  IV., 
V.,  VI.,  VII.  and  VIII.  (1909  to  1913). 


BRICKS 
CHAPTER   XI 

THE    RAW    MATERIALS    FOR    BRICKS 

BRICKS  are  ordinarily  made  of  clay  or  shale,  or  of  a  mixture 
of  these  with  sand.  Hence,  the  raw  materials  used  in  brick- 
making  include  all  the  clays  possessing  such  a  degree  of  plas- 
ticity as  will  enable  them  to  be  formed  into  bricks,  which  are, 
at  the  same  time,  sufficiently  cheap  to  make  the  manufacture 
of  bricks  reasonably  profitable.  Bricks  are  also  made  of  sand, 
as  described  in  a  later  chapter,  and  these  are  known  as  "  sand 
lime  bricks." 

Bride  Clays. — No  rigid  scientific  definition  of  brick  clays 
can  be  formulated,  as  almost  all  clays  can  (technically)  be 
made  into  bricks.  Whether  a  particular  clay  can  be  com- 
mercially used  for  this  purpose  depends  on  a  variety  of  other 
considerations,  the  chief  of  which  is  the  saleable  value  of  the 
clay  for  more  remunerative  purposes.  Thus,  a  fireclay — 
valuable  for  its  ability  to  resist  high  temperatures — may  be 
used  for  the  manufacture  of  building  bricks  in  a  district  where 
the  demand  for  the  latter  is  proportionately  greater  than  that 
for  furnace  bricks.  Again,  some  clays — as  London  clay- 
make  excellent  bricks  when  mixed  with  a  suitable  proportion 
of  sand,  but  large  quantities  are  rendered  quite  useless  for  this 
purpose  by  the  absence  of  sand  from  the  localities  in  which 
they  occur,  and  by  the  selling  price  of  bricks  being  too  low  to 
admit  of  the  transportation  of  sand  from  other  districts. 

For  the  manufacture  of  bricks  it  is  essential  that  the  clay 
shall  not  contract  unduly  during  drying,  so  that  clays  with  a 
high  degree  of  shrinkage  are  excluded  from  use  as  brickmaking 
materials  unless  a  sufficient  quantity  of  sand  is  available  at 
a  cost  not  exceeding  that  of  the  clay,  so  that  from  the  two 
materials  a  mixture  with  the  desired  degree  of  shrinkage  may 
be  obtained. 


THE  RAW  MATERIALS  FOR  BRICKS  287 

For  brickmaking  purposes,  clays  are  classified  according 
to  certain  of  their  physical  characteristics,  and  not  according 
to  their  chemical  composition.  Indeed,  the  latter  is  of  very 
small  significance,  except  in  so  far  as  it  enables  the  physical 
properties  of  the  clays  to  be  predicted.  The  difficulties 
attending  the  interpretation  of  a  chemical  analysis  of  clay 
are,  moreover,  so  extraordinarily  great  that  it  is  often  best  for 
the  brickmaker  to  discard  it  altogether  and  to  rely  upon 
physical  tests. 

The  chemical  composition  of  the  clays  used  for  brickmaking 
is,  in  fact,  so  complex  that  it  is  doubtful  whether  it  is  even 
approximately  understood  by  some  of  the  most  eminent 
mineralogists  and  chemists  at  the  present  time.  Some  indica- 
tion of  the  present  state  of  knowledge  on  this  subject  is  given 
on  pp.  5,  40,  et  seq,  but  the  large  proportions  of  sand  and  other 
substances  found  in  most  brickmaking  earths  make  it  almost 
impossible  to  investigate,  with  accuracy,  the  real  nature  of  the 
true  clays  they  contain.  Fortunately,  for  the  building  and 
allied  trades,  this  lack  of  knowledge  of  the  constitution  of  the 
clay  molecule  does  not  seriously  affect  the  brickmaker,  as  he 
deals  with  the  physical  rather  than  the  chemical  properties 
of  clays. 

For  the  production  of  good  bricks,  a  clay  must,  when  burned, 
possess  a  suitable  colour,  hardness  and  porosity  ;  it  must  be 
accurate  in  shape,  free  from  warping,  and  of  sufficient  strength 
to  carry  any  loads  likely  to  be  placed  upon  it.  In  addition 
to  this  it  must  be  sufficiently  refractory  to  withstand  the 
action  of  any  heat  to  which  it  may  be  subjected.  In  order 
to  produce  good  bricks,  clays  must,  therefore,  be  of  such  a 
nature  when  raw  that  they  can  possess  these  properties  when 
burned. 

The  colour  of  a  raw  clay  is  of  little  or  no  importance  to  the 
brickmaker  and  need  not  be  described  further.  The  colour  of 
the  burned  clay,  on  the  contrary,  is  often  of  the  greatest 
importance,  especially  where  re*d  bricks  are  required  for  use 
on  residential  property  or  blue  bricks  are  needed  for  engineering 
works.  Not  a  few  clays  are  excellent  so  far  as  the  technical 
manufacture  of  bricks  is  concerned,  but  the  bricks  produced 
from  them  have  so  unpleasant  a  colour  that  the  clays  themselves 


288  THE  RAW  MATERIALS  FOR  BRICKS 

are  practically  valueless.  In  some  instances  such  clays  may 
be  purified  or  otherwise  treated  so  as  to  remove  the  discolouring 
materials,  but  in  other  cases  this  treatment  may  prove  so 
expensive  as  to  be  impractical. 

So  important  is  the  colour  of  the  burned  clays,  i.e.,  of  the 
bricks  and  other  goods  produced  from  them,  that  it  is  con- 
venient to  classify  clays  according  to  the  colour  of  the  finished 
goods. 

Red-burning  clays  are  those  from  which  most  of  the  building 
bricks  used  in  the  Midlands  and  the  North  of  England  and  the 
facing  bricks  of  the  South  are  produced.  When  free  from 
discolouring  substances,  the  bricks  made  from  these  clays  are 
of  a  uniform  "  terra-cotta  "  colour,  but  unless  made  from 
carefully  selected  materials  and  prepared  and  burned  with  the 
greatest  care,  some  irregularity  in  colour  is  sure  to  occur. 

The  red  colour  is  usually  attributed  to  ferric  oxide  (red 
oxide  of  iron)  distributed  throughout  the  material  in  the  form 
of  so  fine  a  powder  that  no  artificially  prepared  ferric  oxide 
can  be  used  to  replace  it  or  to  convert  another  class  of  clay 
into  a  really  satisfactory  red-burning  one.  The  amount  of 
ferric  oxide  which  must  be  present  in  a  clay  in  order  to  produce 
a  good,  red  brick  depends  more  on  the  fineness  of  the  oxide 
than  on  the  actual  proportion  present — a  very  small  quantity 
of  an  extremely  fine  powder  exercising  a  far  greater  colouring 
effect  than  a  much  larger  proportion  of  a  coarser  powder. 
Clays  which  show  less  than  5  to  6  per  cent,  of  ferric  oxide  on 
analysis  seldom  produce  bricks  of  a  pleasing  red  colour, 
though  as  little  as  3  per  cent,  will  sometimes  produce  a  good 
red  brick. 

There  is  good  reason  to  suppose  that  no  ferric  oxide  occurs 
as  such  in  the  raw  clay,  but  that  the  iron  is  present  in  the  form 
of  an  almost  colourless  hydro-ferrosilicate  or  ferrosilicic  acid, 
such  as  nontronite  (Fe.f) ^2Si0.22H.20)  which,  on  heating  to  dull 
redness,  decomposes  into  free  ferric  oxide,  silica  and  water. 
Limonite — a  yellow  hydrated  oxide  of  iron — also  occurs  in 
many  clays,  and  on  heating  to  redness  it  also  forms  red  ferric 
oxide. 

C.  F.  Binns  and  others  consider  that  the  colour  of  red  bricks 
and  terra-cotta  is  due  to  colloidal  iron  oxide  having  been 


RED-BURNING  CLAYS  289 

precipitated  in  the  raw  clay.  They  attribute  the  formation 
of  this  colloidal  matter  to  the  oxidation  of  pyrites  with  ferrous 
sulphate,  the  latter,  being  soluble,  permeating  the  clay  and, 
on  further  oxidation,  precipitating  oxide  of  iron.  Alterna- 
tively, any  ferrous  carbonate  present  in  the  clay  may  be 
dissolved  by  water  containing  free  carbonic  acid,  and  after- 
wards oxidised  to  ferric  hydrate  or  oxide.  Ferrous  carbonate 
(siderite)  when  mixed  with  clay  does  not  produce  a  good  red 
brick. 

It  must  be  clearly  understood  that  the  red  colour  is  due  to 
the  action  of  heat  on  the  clays,  as  some  clays  which  are  red, 
when  first  dug,  do  not  make  good  red  bricks,  but  usually 
produce  unpleasantly  discoloured  ones. 

Purple  bricks  are  produced  by  the  reducing  action  of  some 
substance  on  the  iron  compound,  whereby  a  bluish  ferrous 
silicate  is  produced.  The  most  usual  reagent  for  this  purpose 
is  carbon,  which  may  occur  naturally  in  the  clay  or  may  be 
introduced  in  the  form  of  coal  dust,  cinder  dust  ("  soil  "), 
or  sawdust.  The  addition  to  the  clay  of  one-fiftieth  of  its 
weight  of  manganese  dioxide,  or  a  rather  larger  proportion  of 
;;  Weldon  mud,"  will  also  produce  a  purple  colour  in  bricks 
made  of  red-burning  clay. 

Blue  bricks  are  more  grey  than  blue  in  colour.  Like  the 
purple  bricks  just  mentioned,  the  colour  is  produced  by  the 
reduction  of  the  iron  in  a  red-burning  clay  to  ferrous  silicate, 
on  account  of  the  presence  of  some  form  of  carbon  in  the  clay 
or  of  a  strongly  reducing  atmosphere  in  the  kiln.  The  clays  used 
for  blue  bricks  must  be  specially  suitable  for  the  purpose,  and 
must  contain  a  sufficiently  large  proportion  of  iron  (corre- 
sponding to  at  least  5  per  cent,  of  ferric  oxide)  together  with 
sufficient  lime  and  alkalies  to  form  a  brick  which  vitrifies 
easily  without  losing  its  shape  in  the  kiln.  The  so-called  marls 
of  Staffordshire  are  specially  useful  for  the  production  of  blue, 
bricks,  and  with  them  the  blue  colour  is  produced  wholly  in 
the  firing  of  the  kiln.  Some  other  clays  of  an  entirely  different 
composition  and  origin  may  be  converted  into  blue  bricks  by 
special  methods,  such  as  injecting  heavy  oil  into  the  kiln  just 
before  finishing  the  firing,  or  by  adopting  other  means  of 
producing  a  strongly  reducing  atmosphere. 

c.  U 


290  THE  HAW  MATERIALS  FOR  BRICKS 

Yellow  bricks  are  produced  by  the  action  of  heat  on  clays 
containing  only  a  small  proportion  of  iron.  The  intensity  of 
the  colour  is  sometimes  increased  by  mixing  the  clay  with 
cinder  dust  or  other  combustible  material  containing  sulphur. 
Some  yellow  bricks  are  made  from  clays  containing  a  sufficient 
proportion  of  iron  to  burn  red  were  it  not  for  the  simultaneous 
presence  of  chalk  in  the  clay  (see  "  White  Bricks  ").  The 
malm  bricks,  which  are  considered  in  the  London  district  to 
be  the  best  building  bricks,  owe  this  yellow  colour  to  their 
production  from  a  mixture  of  red-burning  clay,  chalk  and 
cinder  dust  ("  soil  "),  the  bricks  being  burned  in  contact  with 
the  fuel  in  a  clamp.  "  London  stocks  "  are  prepared  in  the 
same  manner,  but  are  regarded  as  slightly  inferior  in  quality 
to  malm  bricks  ;  it  is,  however,  not  unusual  to  include  both 
malms  and  stocks  under  the  term  "  stocks." 

Buff  bricks  are  made  by  heating  clays,  containing  only  a 
small  proportion  of  iron,  in  kilns  and  out  of  direct  contact  with 
fuel.  The  proportion  of  iron  (expressed  as  ferric  oxide)  must 
usually  be  less  than  3  per  cent.,  though  a  somewhat  larger 
quantity  may  be  present  if  it  is  in  the  form  of  iron  pyrites 
(FeS2) which  forms  black  spots  and  not  a  red  colouration.  The 
lower  grade  fire-clays  and  certain  vitrifiable  clays  found  in 
the  neighbourhood  of  Little  Bytham  are  the  best  known 
buff -burning  clays. 

The  cause  of  the  buff  colour  has  not  been  ascertained.  That 
it  is  not  due  merely  to  the  presence  of  a  small  quantity  of  iron 
compounds  may  be  proved  by  mixing  a  red-burning  and  a 
white-burning  clay  together.  The  bricks  made  of  such  a 
mixture  are  not  buff-coloured,  but  pale  red.  There  is  reason 
to  suppose  that  alumina  plays  an  important  part  in  the  forma- 
tion of  the  buff  colour,  but  the  nature  of  its  action  is  still 
unknown.  The  observation  of  Seger,  that  all  buff  clays  contain 
a  large  proportion  of  alumina  and  a  small  proportion  of  iron, 
has  been  confirmed  repeatedly,  and  quite  recently  Binns  and 
Makeley  have  produced  good  buff-burning  test  pieces  by  the 
addition  of  alumina  to  red-burning  clays. 

White-burning  clays  are  of  two  classes  :  (a)  those  which  are 
so  free  from  iron  compounds  that  they  are  naturally  white 
when  drawn  from  the  kilns,  and  (b)  clays  containing  iron 


WHITE-BURNING  CLAYS  291 

compounds  together  with  a  sufficient  proportion  of  chalk 
to  prevent  the  development  of  the  red  colour.  The  first 
class  are  typified  by  the  clays  used  for  domestic  pottery  and 
china  ware,  which  are  too  valuable  to  be  used  for  brickmaking, 
and  the  second  class  by  the  white  Suffolk  bricks  and  the 
bricks  made  from  the  alluvial  clays  in  the  centre  of  Ireland 
and  elsewhere. 

Clays  in  which  the  correct  proportions  of  chalk  and  iron 
occur  naturally  are  found  in  various  parts  of  the  country, 
but  many  white  bricks  are  made  by  adding  a  sufficient  quantity 
of  chalk  to  a  red-burning  clay.  If  such  an  artificial  mixture 
is  examined  during  the  burning  it  will  be  found  to  develop 
a  pale  red  colour  at  a  very  low  temperature,  this  colour  dis- 
appearing at  about  850°  C.  heat  on  account  of  the  interaction 
between  the  iron  compound  and  the  chalk. 

The  amount  of  chalk  required  depends  upon  the  colouring 
power  of  the  iron  compounds  in  the  clay.  Usually  10  per  cent, 
.of  the  weight  of  the  clay  is  ample,  and  for  bricks  intended 
to  withstand  the  action  of  the  weather  more  than  12  per  cent, 
of  chalk  can  seldom  be  used  :  for  inside  work,  on  the  contrary, 
as  much  as  25  per  cent,  of  chalk  may  be  present  in  a  clay. 

White  bricks  are  usually  made  in  localities  where  the  clays 
naturally  contain  chalk,  so  that  the  proportion  of  added  chalk 
is  usually  small.  The  proportion  required  must  usually  be 
ascertained  by  making  bricks  with  different  proportions  of 
clay  and  chalk,  and  observing  which  are  the  best  when  drawn 
from  the  kiln.  An  excess  of  chalk  will  weaken  the  brick  and 
will  prevent  its  being  durable,  whilst  if  too  little  chalk  is  added 
the  clay  will  burn  to  a  pale  red  or  "  salmon  "  colour,  according 
to  the  proportion  of  free  iron  oxide  present. 

Grey  bricks  are  popular  in  some  parts  of  Lancashire,  where 
they  are  considered  superior  to  red  bricks.  As  a  matter  of 
fact,  these  grey  bricks  are  merely  red  bricks  which  have  been 
badly  scummed  by  improper  treatment  during  the  earlier  stages 
of  heating  in  the  kiln.  This  scum  is  purely  superficial  and 
has  nothing  whatever  to  do  with  the  strength  or  other  engineer- 
ing qualities  of  the  bricks,  nor  does  it  indicate  that  the  bricks 
have  been  well  burned.  Indeed,  the  preference  of  some 
architects  and  engineers  for  these  scummed  bricks  is  all  the 

U  2 


292  THE  RAW  MATERIALS  FOR  BRICKS 

more  curious  in  that  it  is  based  on  an  entirely  false  conception 
of  the  cause  of  the  scum. 

In  other  parts  of  the  country  grey  bricks  are  produced  by 
firing  red-burning  clays  under  reducing  conditions  (see  "  Blue 
Bricks  "). 

The  texture  of  a  clay  is  frequently  an  important  indication 
of  its  nature,  though  this  cannot  be  relied  upon  entirely. 

In  respect  of  texture,  clays  are  classified  as  (a)  shales, 
(b)  marls,  (c)  loams,  (d)  boulder  clays,  and  (e)  plastic  clays, 
whilst  a  number  of  clays  which  are  not  comprised  in  this 
classification,  but  are  not  used  in  brickmaking,  bear  special 
names. 

Clay  shales  are  clays  which  have  been  subjected  to  enormous 
pressure  since  their  deposition,  with  the  result  that  they  have 
become  harcj  or  indurated  until  they  resemble  a  soft  stone. 
Some  shales  owe  their  hardness  to  the  interpenetration  of  a 
cementitious  solution  of  a  siliceous  nature. 

It  is  important  to  remember  that-  the  term  "  shale  "  relates 
solely  to  the  texture  and  not  to  the  composition  of  the  material. 
There  are,  in  fact,  many  shales  which  are  almost  devoid  of 
clay,  and  consist  entirely  of  siliceous  matter. 

Shales  are  characterised  by  the  manner  in  which  they  are 
split  up  into  layers  when  tapped  at  right  angles  to  the  line  of 
their  deposition. 

The  clay  shales  are  extensively  used  for  the  manufacture 
of  bricks  by  mechanical  methods,  particularly  in  the  North, 
the  Midlands  and  in  South  Wales.  When  observed  in  situ, 
or  when  first  dug,  these  shales  do  not  appear  to  possess  the 
characteristic  properties  of  clay,  but  when  ground  and  mixed 
with  water  their  indurated  texture  is  destroyed  and  their 
original  argillaceous  characters  are  restored.  When  properly 
selected  and  suitably  treated,  clay  shales  make  first-cla^s 
bricks  and  terra-cotta,  and,  like  most  marine-deposited  clays, 
their  colour  when  burned  is  particularly  rich  and  uniform. 

Marls  or  malms  are  natural  mixtures  of  clay  and  chalk, 
and  may  usually  be  recognised  by  their  friable  nature,  their 
texture  being  quite  different  from  that  of  other  clays. 

The  term  "  marl  "  is  sometimes  used  for  any  material  of  a 
friable  nature,  with  a  texture  resembling  the  true  marls, 


CLAY  SHALES  AND  MARLS  293 

but  such  an  extension  of  the  term  is  very  confusing  and  should 
be  avoided.  All  the  true  marls  contain  a  considerable  propor- 
tion of  chalk,  and  therefore  effervesce  violently  when  a  few 
drops  of  hydrochloric  acid  are  allowed  to  fall  on  them.  The 
so-called  marls  of  the  Midlands  and  some  parts  of  Wales, 
on  the  contrary,  are  almost  free  from  chalk  and  are  harder 
than  the  true  marls.  They  are  used  for  blue  bricks. 

True  marls  are  largely  used  for  white  and  yellow  (stock) 
bricks,  their  composition  and  colour  when  burned  being 
modified,  where  necessary,  by  the  addition  of  a  further  pro- 
portion of  chalk.  Clays  to  which  an  addition  of  chalk  has 
been  made  are  frequently  termed  artificial  malms. 

As  it  is  necessary  that  no  coarse  grains  of  chalk  should  be 
mixed  with  the  clay,  the  material  must  be  treated  in  a  wash- 
mill  in  order  to  remove  all  the  larger  particles.  Unless  this  is 
done,  the  larger  particles  of  chalk  are  liable  to  split  the  bricks 
(see  "  Boulder  Clays  "). 

The  true  marls  chiefly  occur  in  the  neighbourhood  of  the 
chalk  deposits  in  the  south  eastern  counties,  but  the  red  marls 
—which,  as  already  explained,  do  not  contain  chalk — occur 
chiefly  in  the  Midlands,  in  North-east  Yorkshire  and  in  Wales. 
The  Midland  marls  belong  to  the  Triassic  clays,  and  are  famous 
for  the  excellent  terra-cotta  colour  of  the  wares  produced  from 
them.  Ruabon  terra-cotta  and  Leicester  red  bricks  and  tiles 
are  typical  products  of  the  Midland  red  marls. 

Further  particulars  respecting  marls  will  be  found  in  the 
author's  "  British  Clays,  Shales  and  Sands  "  (Charles  Griffin 
and  Co.,  Ltd.,  London). 

Loams  are  essentially  mixtures  of  sand  and  clay,  and  if 
stirred  up  with  water  and  allowed  to  settle  for  a  few  minutes 
leave  a  residue  readily  recognised  as  sand.  The  term  loam 
is,  however,  applied  in  a  larger  sense  to  any  earth  which  is 
not  distinctly  a  sand  or  clay,  but  appears  to  partake  of  the 
character  of  both.  Thus,  the  red  marls  of  the  Midlands  (supra) 
are  loamy  in  character,  whilst  not  possessing  the  peculiar 
texture  of  a  true  loam,  and  consequently  loams  and  marls 
are  frequently  confused.  The  distinction  is  clear  when  the 
composition  of  each  is  considered,  and  confusion  is  only  the 
result  of  regarding  the  terms  marl  and  loam  as  having  reference 


294  THE  RAW  MATERIALS  FOR  BRICKS 

exclusively  to  texture.  Even  then,  there  is  a  distinct  difference 
in  texture  between  a  loam  and  a  marl,  though  this  is  often 
overlooked. 

Loams  or  sandy  clays  are  of  great  value  for  brickmaking, 
as  they  do  not  shrink  unduly  and  yet  have  sufficient  plasticity 
to  be  made  into  bricks  and  other  simple  shapes.  Where  the 
proportion  of  clay  in  a  loam  is  less  than  one-quarter  of  the 
total  material  the  loam  is  seldom  suitable  for  brickmaking, 
unless  it  is  mixed  with  a  plastic  clay.  Consequently,  sandy 
loams  are  of  minor  value  to  brickmakers  except  as  a  diluent, 
though  the  addition  of  washed  chalk  will  sometimes  enable 
excellent  bricks  to  be  produced,  the  chalk  acting  as  a  binding 
agent. 

Loams  are  frequently  found  in  association  with  gravel ; 
the  latter  must  be  removed  before  the  loams  can  be  satisfactorily 
used  for  brickmaking.  Merely  to  crush  the  gravel  to  powder  is 
not  sufficient,  unless  the  loam  is  rich  in  clay. 

Boulder  clays  form  a  considerable  part  of  the  glacial  drift 
deposited  over  the  whole  of  the  North  and  Midlands,  and  are 
a  product  of  the  Great  Ice  Age.  The  term  "  boulder  clays  " 
is  sometimes  erroneously  used  for  the  whole  deposit  ;  it  ought 
really  to  be  confined  to  the  argillaceous  portions,  the  term 
applied  to  the  whole  being  drift,  or  more  correctly,  glacial  drift. 
Boulder  clays  are  characterised  (as  their  name  implies)  by 
their  association  with  boulders  and  smaller  stones,  many  of 
these  consisting  of  limestone  brought  by  the  ice  over  very 
great  distances,  and  the  occurrence  of  characteristic  stones 
in  or  close  to  the  clay  affords  on 3  of  the  readiest  means  of 
identifying  it.  It  would  be  too  sweeping  an  assertion  to  state 
that  all  stony  day  is  of  glacial  origin,  but  the  statement  is 
sufficiently  truthful  to  cover  the  majority  of  cases  where 
a  plastic  clay  in  Great  Britain  contains  stones. 

Boulder  clays  are  usually  lightly  plastic  and  tough,  if  care 
be  taken  to  select  portions  free  from  stone.  Some  of  them 
shrink  greatly  in  drying,  and  require  to  be  mixed  with  sand  in 
order  to  produce  sound  bricks.  The  presence  in  them  of  stones 
too  small  to  be  removed  readily  by  hand  picking  is  a  serious 
disadvantage  to  the  brickmaker  desirous  of  using  this  material. 
He  must  either  remove  the  stones  and  gravel  completely,  by 


BOULDER  CLAYS  295 

washing  the  clay,  or  he  must  have  the  larger  stones  removed 
by  hand  or  some  simple  mechanical  means,  and  must  then  crush 
the  gravel  and  smaller  stones  to  so  fine  a  powder  that  they 
can  do  no  further  harm.  Unfortunately,  both  these  methods 
add  largely  to  the  cost  of  brickmaking,  and  many  firms  using 
boulder  clay  and  other  stony  clays  endeavour  to  economise 
by  crushing  and  mixing  the  whole  of  the  material  without 
subjecting  it  to  any  preliminary  cleaning  or  freeing  from 
stones.  Where  the  proportion  of  stones  is  small  and  they  are 
of  a  siliceous  character,  the  number  of  bricks  spoiled  by  a  simple 
crushing  of  the  raw  material  may  be  insignificant,  but  if 
limestone  is  present,  so  large  a  percentage  of  the  bricks  produced 
will  be  cracked,  "  blown  "  and  spoiled,  that  the  matter  becomes 
exceedingly  serious. 

When  a  particle  of  limestone  is  subjected  to  the  ordinary 
heat  of  a  brick  kiln  it  loses  carbon  dioxide  and  is  converted 
into  quicklime.  If  this  particle  of  quicklime  is  situated  at 
or  near  the  surface  of  a  brick  it  gradually  absorbs  water  from 
the  atmosphere,  becomes  hydrated  and  swells,  thereby  exerting 
so  great  a  pressure  that  the  surface  of  the  brick  may  be  cracked. 
If  the  lime  is  on  the  surface  of  the  brick  the  hydrated  product 
forms  a  loose  white  powder,  and  the  brick  is  then  said  to  be 
"  blown."  If  the  particles  of  lime  are  so  far  from  the  surface  that 
they  cannot  crack  or  weaken  the  brick,  little  or  no  harm  is  done 
and,  generally  speaking,  the  action  of  all  particles  of  lime  at  a 
greater  depth  than  one  inch  from  the  surface  may  be  ignored. 

The  only  true  remedy  for  these  defects  is  the  prevention  of 
their  occurrence  by  the  removal  of  the  limestone  previous  to 
the  clay  being  made  into  bricks.  Unfortunately,  this  is,  in 
most  cases,  impossible,  though  much  may  be  done  by  passing 
the  material  through  a  clay  cleaner,  which  consists  of  a 
perforated  drum  through  which  the  soft  clay  is  forced,  whilst 
the  stones  and  gravel  larger  than  the  perforations  remain 
behind.  As  the  perforations  cannot  be  made  less  than  2*5  inch 
in  diameter  if  a  reasonable  output  is  to  be  obtained,  such  an 
appliance  does  not  remove  the  smallest  particles  of  limestone  ; 
but  the  proportion  of  those  left  in  the  clay  which  are  able  to 
destroy  the  bricks  is  so  small  as  to  make  the  use  of  such  a 
clay-cleaner  invaluable, 


296  THE  RAW  MATERIALS  FOR  BRICKS 

An  alternative  means  for  preventing  the  effect  of  limestone 
on  clay  consists  in  grinding  the  material  so  fine  that  the 
particles  will  be  distributed  and  so  cannot  cause  the  disintegra- 
tion of  the  bricks.  Such  bricks  are  quite  sound,  but  have  an 
unpleasantly  spotted  appearance,  due  to  the  white  particles 
of  lime.  If  it  is  possible  to  burn  the  bricks  at  a  temperature 
at  which  the  lime  will  combine  with  the  clay  and  form  a  slag, 
the  spottiness  will  be  removed  and  an  even  stronger  brick 
will  be  produced.  Unfortunately,  most  of  the  British  boulder 
clays  cannot  be  heated  to  a  sufficiently  high  temperature 
without  losing  their  shape. 

Notwithstanding  the  defects  caused  by  the  stones  and  gravel 
associated  with  them,  the  boulder  clays  are  very  largely  used 
for  the  manufacture  of  common  bricks,  whilst  facing  bricks 
are  made  in  considerable  quantities  from  selected  portions 
of  these  clays.  If  care  is  taken  in  excavating  a  boulder  clay, 
it  will  usually  be  found  that  the  stones  and  gravel  occur  in 
seams  and  pockets,  and  that  by  careful  oversight  a  clay  may 
be  obtained  which  is  sufficiently  free  from  stones  and  gravel 
to  make  excellent  bricks. 

Plastic  clays  are  the  most  readily  identified  on  account  of 
their  peculiar  texture.  They  are  usually  composed  of  much 
finer  particles  than  are  marls  and  loams,  and  most  of  them  leave 
an  insignificant  residue  when  mixed  with  water  and  rubbed 
on  a  No.  200  sieve.  This  is  not  due  to  any  difference  in  the 
clay  itself,  but  rather  to  the  coarser  nature  of  the  chalk,  sand 
and  other  ingredients  of  less  plastic  clays. 

Plastic  clays  occur  in  almost  every  part  of  the  United 
Kingdom,  and  most  of  them  may  be  used  for  brickmaking, 
providing  that  the  shrinkage  is  not  too  great.  In  some  localities 
this  may  be  reduced  by  the  addition  of  crushed  rock  or  sand  or 
even  of  clay  burned  specially  for  the  purpose,  but  where  none 
of  these  materials  can  be  used,  highly  plastic  clays  are  of  small 
value  for  brick  and  pottery  manufacture. 

The  term  "  plastic  "  is  somewhat  indefinite,  and  no  means 
has  yet  been  found  whereby  plasticity  can  be  adequately 
and  satisfactorily  represented  numerically.  Indeed,  the  term 
plasticity  appears  to  be  almost  incapable  of  exact  definition, 
its  use  by  potters  and  other  clay  workers  being  different  from 


PLASTIC  CLAYS  297 

the  meaning  attached  to  it  by  artists,  engineers,  and  others 
working  with  different  materials.  The  study  of  plasticity  is 
highly  complex  and  technical,  and  as  it  has  been  dealt  with 
exhaustively  in  the  author's  "  British  Clays,  Shales  and  Sands," 
its  discussion  need  not  be  detailed  here. 

Suffice  it  to  say  that  a  plastic  clay  is  one  which  can  be 
formed  into  any  desired  shape  by  means  of  the  fingers  or  light 
tools,  this  shape  being  retained  indefinitely  and  rendered 
permanent  by  heating  the  clay  to  a  suitable  temperature  in 
a  kiln.  The  plasticity  is  thus  seen  to  be  closely  related  to  the 
complexity  of  shape  of  articles  which  can  be  made  from  a 
given  clay.  A  highly  plastic  clay  can  be  formed  into  the  most 
complex  curves  and  patterns,  whilst  a  slightly  plastic  clay  can 
only  be  made  into  simple  forms,  such  as  bricks  and  bars. 
Hence,  clays  are  grouped  according  to  their  plasticity,  the 
highly  plastic  clays  being  termed  tough,  strong  or  foul,  and  less 
plastic  clays  as  lean,  mild  or  weak.  These  terms  are  by  no 
means  exactly  definable,  but  they  serve  to  distinguish  between 
many  different  clays,  and  to  indicate  their  apparent  nature. 

The  apparent  plasticity  of  a  clay  depends  largely  on  the 
condition  in  which  the  clay  is  found,  and  may  vary  from  day 
to  day,  or  even  from  hour  to  hour.  Thus,  a  clay  when  found 
may  appear  to  be  lean  and  almost  devoid  of  plasticity,  yet  if 
suitably  moistened  or  mixed  with  a  little  water  it  will  become 
as  plastic  as  a  highly  plastic  clay.  It  is  therefore  necessary,  in 
describing  the  plasticity  of  a  clay,  to  state  the  conditions  under 
which  the  description  is  applicable,  as  otherwise  the  same  clay, 
on  one  occasion,  may  be  described  as  highly  plastic  and  on 
another  as  of  a  mild  nature — the  state  of  the  atmosphere 
making  a  great  difference  in  the  appearance  of  the  material. 

Many  methods  have  been  proposed  for  comparing  the 
plasticity  of  clays,  but  none  of  them  are  entirely  satisfactory, 
as  this  property  is  not  of  an  elementary  nature,  but  is  closely 
associated  with  the  binding  power,  viscosity,  adhesion,  cohesion, 
absorption,  impermeability,  and  other  properties  of  the 
particles.  Attempts  to  measure  plasticity  will  usually  be 
found,  on  investigation,  to  consist,  in  reality,  in  the  measure- 
ment of  one  of  these  other  properties,  and  not  in  that  of  the 
plasticity  itself.  The  result  is  that  numbers  supposed  to 


298  THE  RAW  MATERIALS  FOR  BRICKS 

represent  plasticity  obtained  by  different  investigators  differ 
widely  from  each  other  and,  which  is  more  important,  lead  to 
conclusions  inconsistent  with  what  is  ordinarily  understood  as 
plasticity  by  practical  clay  workers.  Thus,  the  plasticity 
numbers  of  a  certain  chemist  in  London  represent  china  clay 
as  less  plastic  than  ball  clay,  whereas  all  potters  hold  precisely 
the  reverse  opinion. 

If,  as  appears  probable,  the  plasticity  of  clay  is  due  to  a 
variety  of  minor  properties,  it  can  only  be  adequately  measured 
by  some  means  in  which  all  these  minor  properties  are  included . 
Such  a  method  would  appear  to  be  very  complex,  and  almost 
impossible  in  its  entirety.  The  plasticity  of  a  clay  may,  how- 
ever, be  measured  with  an  accuracy  sufficient  for  most  practical 
purposes,  as  follows  : — 

(a)  A  sample  of  clay  is  mixed  with  sufficient  water  to  form  a 
paste  of  good  modelling  consistency ;  that  is  to  say,  a  paste 
which  is  sufficiently  moist  to  be  formed  into  any  desired  shape 
and  yet  not  so  moist  as  to  adhere  to  the  fingers.  A  weighed 
portion  of  this  paste  is  then  dried  at  105°  C.  and  the  proportion 
of  water  to  dry  clay  is  ascertained.  A  less  accurate  method 
consists  in  weighing  the  clay  and  measuring  the  water  required 
to  mike  a  paste  of  the  desired  consistency,  but  allowance 
must  then  be  made  for  the  proportion  of  moisture  in  the 
original  clay.  Clay  which  has  been  dried  artificially  must  not 
be  used  for  this  test,  unless  the  plasticity  of  such  dried  clay  is 
to  be  determined  instead  of  that  of  the  raw  clay. 

(6)  Some  of  the  clay  paste  produced  as  just  described,  is 
placed  in  a  small  expression  machine  (a  sausage  mincer  of  the 
old-fashioned  type  being  suitable),  and  in  this  way  a  cylinder 
of  clay  paste  about  three  inches  long  and  one  inch  diameter 
is  formed.  This  cylinder  is  fitted  with  clamps  at  each  end, 
and  two  marks  exactly  two  inches  apart  are  then  made  on  it. 
Its  tensile  strength  is  determined  by  attaching  one  clamp  to 
a  support  and  applying  a  suitable  weight  to  the  other.  The 
weight,  which  is  exactly  sufficient  to  rupture  the  cylinder,  is 
noted,  the  two  broken  pieces  of  clay  are  then  fitted  together, 
and  the  percentage  of  extension  in  length  is  then  measured. 

The  product  of  the  percentage  of  water  required  in  (a),  the 
percentage  extension  and  the  tensile  strength  expressed  in 


PLASTICITY  299 

kilogrammes  per  square  centimetre  is  a  figure  which  has  been 
shown  by  Zschokke,  Rasenow  and  others  to  agree  with  the 
relative  plasticity  of  clays  as  far  as  this  can  be  judged  by 
practical  potters. 

This  method  is  imperfect,  inasmuch  as  it  does  not  give 
sufficient  prominence  to  the  power  of  certain  clays  to  retain 
their  plasticity  when  mixed  with  sand  or  rock-dust,  but  it 
has  proved  in  the  author's  experience  to  be  the  least  objec- 
tionable of  any  method  yet  published,  and  its  value  may 
readily  be  increased  by  applying  it  to  mixtures  of  any  clay  to 
be  tested  with  sand  or  other  non-plastic  material. 

In  the  production  of  bricks  and  tiles  the  plasticity  of  a  clay 
is  seldom  developed  to  its  fullest  extent.  Indeed,  to  do  this 
would  usually  render  the  clay  useless  for  these  particular 
purposes,  as  the  paste  would  shrink  so  much  in  drying  and 
burning  as  to  warp  or  crack.  Where  pottery  or  terra-cotta  is 
being  manufactured  a  higher  degree  of  plasticity  is  usually 
necessary,  and  is  obtained  by  a  more  thorough  mixing  with 
water  and  by  storing  the  clay  paste  under  conditions  likely  to 
increase  its  plasticity.  This  "  souring  "  or  "  maturing  "  is  a 
slow  process,  and  some  porcelain  clay-mixtures  require  to  be 
stored  for  several  years  before  they  are  fit  for  use. 

The  causes  of  plasticity  being  imperfectly  understood,  it  is 
clear  that  the  best  method  of  increasing  the  plasticity  of  lean 
clays  is  still  not  known  with  certainty.  If,  as  appears  probable, 
the  plasticity  of  clays  is  due  to  the  hydrolytic  action  of  water 
on  the  clays  themselves,  it  is  probable  that  any  increase  in 
plasticity  must  be  effected  by  increasing  this  action  of  water. 
If,  as  suggested  by  Rohland  and  others,  plasticity  is  a  charac- 
teristic of  colloidal  substances  mixed  with  inert  particles,  it 
will  be  increased  by  any  treatment  which  will  increase  the 
proportion  of  colfoids,  e.g.,  by  storing  in  a  moist  condition  in 
a  cool  place.  An  artificial  or  pseudo-plasticity  may  be  con- 
ferred on  clay  by  the  addition  of  various  vegetable  colloids, 
such  as  starch  paste,  or  of  other  colloids,  such  as 
glue,  but  the  use  of  such  materials  is  seldom  practicable  in 
brickmaking,  as  it  is  too  costly. 

The  reduction  of  the  plasticity  of  clays  may  be  effected  in 
several  ways,  of  which  the  most  important  are  :  (a)  drying, 


300  THE  RAW  MATERIALS  FOR  BRICKS 

(6)  heating  to  redness,  (c)  mixing  with  sand  or  other  inert 
material,  and  (d)  by  the  addition  of  a  minute  quantity  of  a 
suitable  chemical.  Of  these  methods,  drying  is  only  temporary, 
the  clay  again  becoming  plastic  on  further  treatment  with 
water.  Heating  to  a  temperature  short  of  redness  will  reduce 
the  plasticity  of  clay  in  proportion  to  the  amount  of  water 
removed.  If  only  the  free  water  is  driven  off,  the  clay  will  again 
become  plastic  when  moistened,  but  not  if  the  clay  molecule 
has  been  partially  destroyed  as  only  the  complete  molecules  can 
become  plastic.  When  a  clay  has  been  heated  to  redness  it 
cannot  again  become  plastic  ;  its  chemical  constitution  has 
been  destroyed  and  it  is  no  longer  a  "  clay  "  (p.  40). 

Clays  which  have  had  their  plasticity  reduced  by  mixing 
with  sand  may  have  it  restored  by  any  simple  process  of 
elutriation  or  washing  which  will  remove  the  added  material, 
but  those  which  have  been  treated  with  chemicals  behave 
differently.  Thus,  the  addition  of  a  few  drops  of  baryta 
solution,  or  of  a  solution  of  caustic  soda  or  potash,  will  convert 
a  stiff  plastic  paste  into  a  fluid  slip  or  cream,  but  the  addition 
of  a  quantity  of  acid  just  sufficient  to  neutralise  the  alkali 
previously  added  will  re-convert  the  fluid  slip  into  a  stiff 
paste. 

The  effect  of  merely  a  few  drops  of  acid  and  alkali  may  be 
shown  in  a  striking  manner  in  the  following  experiment,  which 
is  well  worth  making  by  everyone  interested  in  clays  : — 

Solutions  of  hydrochloric  acid  and  of  caustic  soda  are  prepared 
of  such  a  strength  that  a  given  volume  of  one  of  them  exactly 
neutralises  an  equal  volume  of  the  other.  (The  "  normal  " 
solutions  sold  by  manufacturing  chemists  are  suitable.)  A 
stiff  clay  paste  is  made  by  mixing  -a  little  dry  clay  with  water 
in  a  shallow  cup  or  basin,  and  its  stiffness  noted.  A  few  drops 
(accurately  measured)  of  the  alkaline  solution  are  then  added, 
and  the  mixing  is  continued  until  the  mass  becomes  sufficiently 
fluid  to  pour  into  another  vessel.  When  its  fluidity  has  been 
clearly  demonstrated,  the  same  volume  of  the  acid  solution 
(a  few  drops)  is  added  and  the  mixing  still  further  continued, 
when  it  will  be  found  that  the  clay  again  becomes  stiff  and 
pasty  and  cannot  be  poured  from  one  vessel  to  another. 
Strictly  speaking,  the  addition  of  acid  or  alkaline  solutions  only 


PLASTICITY  301 

alters  the  viscosity  of  the  clay,  but  this  property  is  so  closely 
allied  to  plasticity  that  anything  which  affects  the  one  must 
have  some  influence — though  not  necessarily  a  proportionate 
one — on  the  other. 

It  has  also  been  suggested  that  the  power  possessed  by  clays 
of  adsorbing  dyes  from  solution  might  be  made  a  measure  of 
plasticity,  but  experience  has  shown  that  adsorption  and 
plasticity  are  not  related  closely  enough  for  this  purpose, 
though  in  many  clays  the  relationship  is  remarkable. 

At  the  present  time  there  is  no  entirely  satisfactory  method 
of  determining  the  plasticity  of  clays,  the  methods  now  in 
use  being  merely  approximations,  some  of  which  are  far  from 
accurate,  except  when  used  to  compare  several  clays  of  the 
same  type. 

The  shrinkage  of  clay  pastes  is  an  important  factor  when 
classifying  clays  with  regard  to  their  suitability  for  brick- 
making.  If  the  volume  of  a  piece  of  clay  paste  is  accurately 
measured  before  and  after  drying,  it  will  be  found  to  have 
diminished  roughly  in  proportion  to  the  plasticity  of  the  clay 
and  to  the  amount  of  water  present  in  the  paste.  If  the  test 
piece  is  then  burned  in  a  kiln  and  is  re-measured  when  cold,  a 
further  reduction  in  volume  will  have  occurred  ;  this  is  known 
as  kiln  shrinkage. 

The  shrinkage  on  drying  which  clay  pastes  undergo  is  due 
to  the  removal  of  the  water  surrounding  (and  possibly  pene- 
trating) each  particle,  with  the  result  that,  as  this  water 
evaporates,  the  particles  are  brought  nearer  together,  until 
finally  they  are  as  close  as  their  shapes  permit.  As  the  particles 
are  irregular  in  shape,  complete  contact  over  the  whole  of 
their  surfaces  is  impossible,-  and  some  pore  spaces  or  voids  are 
bound  to  occur. 

Any  colloidal  material  present  in  the  clay  will  absorb  water 
with  which  it  comes  into  contact — -just  as  dry  glue  swells  when 
immersed  in  water  and  forms  a  bulky  gelatinous  mass.  Hence, 
the  volume  of  the  individual  particles  of  colloidal  matter  in 
the  clay  is  increased  when  it  is  mixed  with  water  and  made 
into  a  paste.  When  the  clay  paste  is  dried,  this  water  is 
removed  and  the  colloidal  particles  shrink  to  their  original 
(dry)  volume.  If  it  be  true  that  the  plasticity  of  clays  is  due 


302          THE  RAW  MATERIALS  FOR  BRICKS 

to  the  proportion  of  active  colloidal  matter  present,  then  the 
shrinkage  they  undergo  on  drying  must  bear  some  relation 
both  to  the  colloidal  matter  and  to  their  plasticity.  Other 
causes  of  shrinkage — such  as  the  surrounding  of  clay  particles 
into  a  film  or  covering  of  water — are  also  present,  and  render 
it  impossible  to  measure  the  influence  of  the  colloidal  matter, 
so  that  the  plasticity  and  shrinkage  are  not  directly  propor- 
tional, yet,  broadly,  the  more  plastic  the  clay  the  greater  will 
be  the  shrinkage  of  the  clay  paste  and  vice  versa. 

Excessive  shrinkage  of  a  clay  paste  involves  so  great  a 
movement  of  the  particles  that  it  is  almost  impossible  to  dry 
some  clays  without  cracking  them.  Even  when  no  heat  is 
used,  the  irregular  rate  at  which  various  parts  of  the  material 
dry  is  a  prolific  cause  of  cracks,  and  one  of  the  most  difficult 
and  worrying  problems  of  the  clayworker  consists  in  drying 
bricks  and  other  articles  at  a  reasonable  speed  without  intro- 
ducing internal  strains  and  stresses  which  result  in  the  fracture 
of  the  articles.  The  problem  is  still  further  complicated  by 
the  fact  that  it  is  often  impossible  to  distinguish  the  cracked 
articles  before  they  are  fixed,  so  that  it  is  only  after  the  com- 
pletion of  the  manufacture — when  they  are  drawn  out  of  the 
kilns — that  the  defective  articles  can  be  separated  from  the 
rest. 

The  shrinkage  which  would  occur  if  the  clay  were  taken 
direct  from  the  pit  and  dried,  is  of  small  importance  to  the 
brick  manufacturer,  and  though  it  is  frequently  reported  in 
tests  of  clays  it  does  not  give  the  information  required,  except 
in  those  cases  where  the  material  is  worked  without  the  addition 
of  any  water.  The  clay  or  other  material  must  first  be  made 
into  a  paste  of  the  consistency  it  will  have  when  moulded  or 
otherwise  shaped  (see  p.  332),  and  it  should  then  be  moulded 
into  the  form  of  a  small  brick  or  bar.  The  internal  measure- 
ments of  the  mould  will  then  give  the  size  of  the  test-piece. 
After  drying,  until  the  whole  of  the  free  water  is  removed, 
the  test-piece  is  again  measured  and  the  shrinkage  calculated 
as  (a)  percentage  of  the  original  volume,  (6)  percentage  of 
the  original  length,  or  (c)  in  inches  per  linear  foot.  All  three 
units  of  shrinkage  are  in  use,  but  the  one  most  commonly 
employed  in  practice  is  the  last,  viz.,  the  reduction  in  length 


THE  SHRINKAGE  OF  CLAY  303 

which  would  be  observed  if  a  bar  or  block  exactly  one  foot 
in  length  were  made  of  the  clay  paste  and  then  dried. 

The  volume  shrinkage  (a)  may  be  roughly  calculated  by 
multiplying  the  lineal  shrinkage  (b  or  c)  by  three,  but  this  is 
not  exact  enough  for  accurate  work. 

It  is  generally  found  that  clays  with  a  shrinkage  exceeding 
one-and-a-half  inches  per  linear  foot  are  unsuitable  for  brick- 
making,  unless  their  shrinkage  can  be  reduced  by  the  addition 
of  a  non-plastic  material.  The  majority  of  brick  manufacturers 
prefer  a  clay  with  a  shrinkage  of  one  inch  per  linear  foot. 
Smaller  shrinkage  usually  indicates  that  the  plasticity  of  the 
clay  is  insufficiently  developed  or  that  too  much  sand  or  other 
inert  material  is  present,  with  the  result  that  the  bricks  will 
probably  be  soft  and  too  easily  crushed  to  be  used  to  more  than 
a  very  limited  extent.  At  the  same  time,  the  fact  must  not 
be  overlooked  that  some  clays  with  an  exceedingly  low  shrink- 
age are  made  into  exceptionally  strong  bricks,  some  special 
reactions  occurring  in  the  kiln  which  overcome  the  weakness 
due  to  lack  of  plasticity  in  the  clay. 

In  the  manufacture  of  architectural  terra-cotta,  in  which 
a  large  number  of  pieces  of  different  shapes  must  fit  accurately 
together,  it  is  highly  important  that  accurate  allowances 
shall  be  made  for  shrinkage.  Where  this  is  not  the  case  the 
pieces  fit  so  badly  that  the  strength  of  the  structure  is  seriously 
reduced,  and  its  aesthetic  value  is  largely  destroyed.  As  the 
calculations  for  shrinkage  are  tedious  when  large  quantities 
of  work  are  to  be  executed,  manufacturers  of  architectural 
terra-cotta  usually  provide  their  modellers  with  specially 
constructed  measuring  scales  by  means  of  which  all  calculations 
for  shrinkage  are  avoided. 

The  shrinkage  of  clays  is  an  interesting  study,  inasmuch  as 
it  occurs  at  some  stages  with  great  regularity,  and  at  others 
in  a  curiously  irregular  manner.  Carefully  recorded  measure- 
ments of  clays  at  various  stages  during  drying  and  burning  seem 
to  show  that  the  shrinkage  proceeds  proportionately  to  the 
loss  of  water  during  the  early  stages  of  drying,  but  as  soon  as 
the  particles  have  come  as  closely  into  contact  with  each  other 
as  their  shape  permits,  shrinkage  ceases,  though  the  loss  of 
water  continues.  This  indicates  the  close  of  the  first  and  most 


304  THE  RAW  MATERIALS  FOR  BRICKS 

difficult  stage  of  drying,  for  it  is  in  the  removal  of  the  greater 
part  of  the  absorbed  or  colloidal  water,  and  the  accompanying 
shrinkage,  that  the  greater  part  of  the  cracking  and  warping  of 
the  goods  occurs.  Some  clays  are  so  delicate  at  this  stage  that 
the  only  way  to  dry  them  is  to  cover  them  closely  with  tarpaulin 
and  to  heat  them  with  wet  steam.  The  moisture  in  the  steam 
prevents  them  drying,  whilst  being  heated,  and  when  they 
are  at  a  sufficiently  high  temperature  for  the  water  in  them 
to  be  converted  into  vapour,  the  steam  supply  is  cut  off  and 
the  bricks  are  uncovered  and  heated  indirectly.  This 
"  sweating  "  is  tedious  and  costly,  but  it  is  the  only  means 
available  with  certain  clays.  '  Most  clays  are  less  troublesome 
and  provided  they  are  protected  from  draughts  or  irregular 
heating,  they  may  be  dried  in  sheds  of  which  the  floors  are 
heated  with  steam  or  fuel,  and  in  summer  large  numbers  of 
bricks  are  dried  in  the  open  air — a  slow  process  requiring 
several  weeks. 

In  the  second  stage  of  the  drying,  when  no  shrinkage 
accompanies  the  removal  of  the  water,  the  goods  may  be  heated 
more  strongly  so  as  to  dry  them  with  fair  rapidity  without 
incurring  any  serious  risk  of  warping  or  cracking.  At  the  end 
of  this  second  stage  the  bricks  or  other  articles  are  placed  in 
the  kiln,  and  there  undergo  a  further  contraction  ("  kiln 
shrinkage  ")  due  to  the  decomposition  of  the  clay  molecule 
and  the  evolution  of  water.  If  the  goods  are  damp  when 
placed  in  the  kiln,  a  further  drying  first  occurs,  and  this 
necessitates  a  very  gentle  warming  of  the  kiln  during  the  first 
two  or  more  days  ;  otherwise  the  evolution  of  steam  will  be 
so  rapid  that  the  goods  will  be  cracked  and  broken  by  the 
expansive  force  of  the  imprisoned  steam.  When  the  heating 
is  sufficiently  slow,  the  steam  escapes  steadily,  and  without 
doing  any  damage,  leaving  a  porous  material  devoid  of  all 
plasticity  and  resembling  an  extremely  soft  stone. 

The  action  of  heat  on  clays  to  be  used  for  bricks  is  also 
important.  Some  clays  contain  so  large  a  proportion  of  metallic 
salts  and  oxides  other  than  alumina  and  silica,  that  they  cannot 
be  burned  on  a  large  scale  without  serious  loss  of  shape.  The 
precise  changes  which  occur  when  clay  is  heated  must  be  left 
to  a  later  chapter;  it  is  sufficient  for  the  moment  to  state 


ACTION  OF  HEAT  ON  CLAYS  305 

that  a  satisfactory  clay  for  brickmaking,  etc.,  must  be  able 
to  withstand  the  heat  of  a  kiln  sufficiently  long  for  a  hard, 
compact  material  of  the  desired  characteristics  to  be  produced. 
To  some  extent  the  temperatures  reached  in  the  kilns  may 
be  adjusted  to  suit  the  clays  being  heated,  but  this  can  only 
be  done  to  a  limited  extent,  and  where  the  composition  of  the 
material  made  into  bricks  is  very  irregular,  it  will  be  difficult, 
or,  in  some  cases,  impossible,  to  secure  a  large  proportion 
of  saleable  goods. 

When  overheated,  bricks  become  vitrified,  slag-like  and  very 
irregular  in  shape.  They  are  deficient  in  porosity  and,  conse- 
quently, are  very  difficult  for  the  bricklayer  to  use.  Such 
bricks  are  known  by  a  variety  of  names — many  of  them  of 
purely  local  use — such  as  crozzles,  burrs  and  clinkers. 

Firebricks  are  distinguished  from  others  by  their  exceptional 
resistance  to  high  temperatures.  Most  red-burning  bricks 
will  run  to  a  shapeless  mass  if  maintained  for  several  hours 
at  a  temperature  of  1,200°  C.,  but  firebrick  will  not  lose 
its  shape  at  1,600°  C.,  and  the  better  qualities  are  in  regular 
use  at  much  higher  temperatures.  It  is  almost  impossible 
to  overheat  firebricks  during  manufacture,  but  the  clays  from 
which  they  are  made  are  too  valuable  to  be  used  for  ordinary 
building  bricks. 

Impurities  in  Clays. — Having  realised  that  there  are  a  number 
of  ways  of  classifying  clays  according  to  one  or  more  of  their 
important  properties,  the  reader  will  readily  understand  that 
an  exact  and  complete  classification  is  at  present  unattainable. 
All  that  can  be  stated  is  that  most  clays  appear  to  consist  of 
mixtures  of  stones,  gravel,  sand,  silt,  rock-flour  and  other 
inert  materials — all  of  which  are  obviously  not  of  the  nature 
of  clay — with  a  substance  of  a  more  or  less  plastic  nature, 
the  composition  of  which  resembles  that  of  an  aluminosilicic 
acid  (p.  40). 

According  to  the  impurities  present,  a  clay  will  be  useful 
or  worthless  for  certain  purposes,  and  no  single  clay  can  be 
equally  valuable  for  ail  the  purposes  for  which  clays  are  used. 
Thus,  an  almost  pure  clay  would  produce  white  bricks,  the 
production  of  red  bricks  and  terra-cotta  necessitating  the 
presence  of  certain  impurities,  notably  iron  compounds.  Again, 

c<  x 


306  THE  RAW  MATERIALS  FOR  BRICKS 

a  pure  clay,  if  plastic,  would  shrink  too  much  for  ordinary  use 
and  would  have  to  be  diluted  with  sand  or  other  non-plastic 
material.  Hence,  pure  clay — which  does  not  occur  in  Nature — 
is  of  less  value  to  the  brickmaker  than  the  impure  clays  which 
serve  his  purposes  so  admirably.  "Pure  clay"  is,  in  fact, 
little  more  than  an  abstract  idea,  for  the  most  carefully  refined 
plastic  clays  are  invariably  so  different  in  properties  from  the 
refined  china  clays  of  apparently  the  same  composition  as  to 
leave  it  a  matter  of  conjecture  whether  they  are  two  forms  of 
identically  the  same  substance  or  not.  What  appears  to  be 
most  probable  is  that  they  are  different  aluminosilicic  acids  of 
highly  complex  structure. 

Whatever  may  be  the  real  nature  of  the  essential  substance 
of  all  "  clays  "  and  brick  earths,  it  is  important,  in  some  cases, 
to  ascertain,  at  least  roughly,  the  amount  of  clay  substance 
present.  This  is  a  matter  for  the  expert  in  clays  and  cannot 
be  ascertained  by  the  ordinary  clayworker,  nor  is  it  shown 
by  the  analysis  made  by  public  analysts  and  other  chemists 
with  no  special  knowledge  of  clays.  A  method  of  so-called 
"  rational  analysis  "  sometimes  used  for  this  purpose  is  particu- 
larly misleading  and  erroneous  when  applied  to  brickmaking 
materials. 

It  is  convenient,  and  for  most  purposes  sufficiently  accurate, 
to  consider  clays  and  brick  earths  as  being  composed  of  a  certain 
amount  of  real  clay a  together  with  silt,  sand,  chalk,  gravel  and 
stones.  According  to  the  nature  and  proportions  of  these 
adventitious  ingredients  the  clays  will  prove  satisfactory  or 
otherwise  for  the  manufacture  of  any  given  articles. 

In  addition  to  the  occurrence  of  impurities  in  these  recog- 
nised forms,  however,  clays  also  contain  impurities  which  are 
more  conveniently  considered  under  their  separate  names 
rather  than  as  constituents  of  sand,  etc.  The  most  important 
of  these  impurities  are  : — 

Free  silica  present  as  sand,  or  in  a  colloidal  form  readily 

1  The  author  has  suggested  the  word  pelinite — derived  from  a  Greek  word  mean- 
ing "  of  the  nature  of  clay  " — for  the  clayey  substance  found  in  all  plastic  clays. 
The  identity  or  otherwise  of  pelinite  and  kaolinite — the  latter  being  the  essential 
constituent  of  kaolins  and  china  clays — remains  to  be  proved,  though  the  relation- 
ship between  the  two  appears  to  be  very  close.  Kaolinite  is,  however,  almost 
devoid  of  plasticity.  (See  the  author's  "  Natural  History  of  Clay,"  Cambridge 
University  Press.) 


IMPURITIES  IN  CLAYS  307 

soluble  in  a  solution  of  caustic  soda.  This  colloidal  silica 
absorbs  water  readily  and  shrinks  greatly  on  drying,  and  so 
is  liable  to  cause  trouble  in  manufacture.  The  free  silica 
present  as  grains  of  quartz  or  sand,  merely  serves  as  a  diluent 
of  the  clay,  reducing  its  plasticity  and  shrinkage.  If  these 
silica  grains  are  impure  they  will  affect  the  clay  according  to 
the  impurities  they  contain  (see  later). 

If  a  clay  is  very  impure  and  readily  fusible,  the  addition  of 
sand  may  increase  its  power  of  heat-resistance  as  free  silica  is, 
in  itself,  highly  refractory. 

Free  alumina  is,  in  many  respects,  like  free  silica  so  far 
as  its  behaviour  in  brick  clays  is  concerned.  It  increases  the 
heat-resistance  of  clays  containing  it,  and  appears  to  aid  in 
the  formation  of  buff -coloured  bricks.  Free  alumina  occurs 
in  British  clays  to  so  small  an  extent  that  it  is  of  little  import- 
ance, but  in  some  tropical  countries  its  presence  is  highly 
significant. 

Lime  compounds  occur  chiefly  in  the  form  of  calcium  car- 
bonate (limestone  and  chalk)  and  calcium  sulphate  (gypsum). 
Both  these  substances  are  converted  into  quicklime  on  pro- 
longed heating  in  a  brick  kiln,  but  the  conversion  is  not  always 
complete.  Lime  may  also  occur  in  the  form  of  a  calcium 
alumino-silicate  as  a  species  of  felspar.  It  then  acts  as  a  flux, 
binding  the  surrounding  particles  together  as  described 
under  (a)  below. 

Lime  compounds  have  three  important  characteristics 
when  they  occur  in  clays  : — 

(a)  On   heating   the   clay,    the   lime   unites   with   the   clay 
substance  and  with  any  free  silica  or  free  alumina  present, 
and  forms  a  viscous  glassy  mass,  which  cements  the  less  fusible 
particles  together  into  a  hard  vitrified  and  very  strong  mass. 
Where   sufficient  lime  is  present  the  mass  may  become   so 
viscous  as  to  lose  its  shape  and  form  burrs,  or  clinkers  (p.  377) 
instead  of  well  shaped  and  sound  bricks. 

(b)  Soluble  lime  compounds  (chiefly  the  sulphate)  dissolve 
in  the  water  used  in  making  the  clay  paste,  and  are  brought 
to  the  surface  of  the  goods  during  the  drying.     As  the  water 
evaporates  it  leaves  the  salts  on  the  surface  in  the  form  of  a 
thin  white  deposit  or  scum,  which  is  much  more  noticeable 

X  2 


308  THE  RAW  MATERIALS  FOR  BRICKS 

on  the  burned  than  on  the  green  bricks.  The  only  means  of 
preventing  this  scum  consists  in  converting  the  soluble  salts 
into  insoluble  ones  by  the  addition  of  barium  carbonate  or  some 
other  suitable  chemical  to  the  clay  previous  to  mixing  with 
water.  The  scum  does  no  harm  to  the  bricks  except  that  it 
is  unsightly  and  detracts  from  their  selling  value. 

(c)  Small  nodules  of  limestone,  situated  near  the  surface  of 
a  brick  which  is  not  heated  sufficiently  in  the  kiln  to  cause  a 
combination  of  the  lime  with  the  other  constituents,  will  become 
hydrated  on  exposure  to  moist  air,  and  will  then  swell  and  crack 
the  bricks,  or  they  will  "  blow  "  and  form  ugly  hollow  places. 
This  defect  is  peculiarly  characteristic  of  some  boulder  clays 
(p.  294)  and  many  thousands  of  bricks  are  damaged  annually 
in  this  manner.  Two  remedies  are  employed  :  ( 1 )  grinding  the 
material  so  fine  that  the  particles  of  lime  will  be  too  small 
to  harm  the  bricks,  together  with  an  increased  finishing 
temperature  in  the  kiln  which  will  cause  them  to  combine  with 
the  silica,  etc.,  in  the  bricks  and  to  become  harmless  ;  (2)  the 
bricks  may  be  immersed  in  water  as  soon  as  they  are  drawn 
from  the  kiln.  This  sudden  slaking  of  the  lime  prevents  the 
bricks  from  cracking,  but  has  the  disadvantage  that  they 
are  made  heavy  with  the  water  they  contain,  and  are  not  so 
readily  purchased  by  builders. 

Magnesium  compounds  so  closely  resemble  those  of  lime 
that  there  is  no  need,  in  brickmaking,  to  distinguish  them. 
In  the  form  of  mica  (a  magnesium  silicate)  magnesium  com- 
pounds occur  in  a  large  variety  of  clays  and  may  usually  be 
recognised  by  a  silvery  sheen,  which  is  characteristic  of  mica 
particles.  There  are  several  kinds  of  mica,  all  of  which  are 
in  the  form  of  very  thin  plates,  which  do  little  or  no  damage 
when  present  in  small  quantities,  but  in  larger  ones  they  tend 
to  weaken  a  structure  of  articles  made  from  the  clay  in  which 
they  occur. 

The  influence  of  mica  in  the  kiln  is  of  small  importance  in 
ordinary  brickmaking  ;  it  tends  to  make  the  clay-mass  fusible, 
but  at  the  ordinary  temperatures  at  which  bricks  are  burned 
its  action  is  scarcely  noticeable. 

Sodium  and  potassium  compounds  are  generally  regarded 
together  under  the  term  alkalies,  though,  in  reality,  the  forms 


IMPURITIES  IN  CLAYS  309 

in  which  they  occur  are  salts,  such  as  felspar,  muscovite,  mica, 
and  other  minerals.  Some  clays  also  contain  sea  salt  (sodium 
chloride),  sodium  sulphate,  potassium  chloride  and  sulphate, 
and  other  soluble  salts.  These  form  a  scum,  as  described  in 
(b),  p.  307. 

Usually,  sodium  and  potassium  compounds  are  among  the 
most  fusible  constituents  of  a  brickmaking  material,  and  they 
therefore  bind  the  more  refractory  particles  together.  When 
present  in  very  small  proportions,  they  increase  the  strength 
of  the  bricks,  but  in  large  proportions  they  bring  about  so 
rapid  a  vitrification  and  fluidity  of  the  clay  that  an  excessive 
number  of  shapeless  goods  is  produced. 

Iron  compounds  in  brick  clays  appear  to  have  been  derived 
from  a  number  of  minerals,  of  which  the  most  important  are  : 
(a)  certain  ill-defined  ferrosilicates  which  form  free  ferric  oxide 
when  the  clay  is  heated,  (b)  pyrites  and  other  forms  of  ferric 
sulphide,  (c)  limonite  and  other  forms  of  ferric  hydrate,  and 
(d)  other  iron  compounds  occurring  in  very  small  quantities, 
and  of  such  a  nature  that  they  may  be  regarded  by  the  brick 
manufacturer  as  though  replaced  by  compounds  in  one  of  the 
three  previous  groups. 

Silicates  containing  iron  are  not  easily  identified  in  clays, 
but  are  of  two  classes — those  in  which  the  iron  remains  in  the 
form  of  a  silicate  after  being  heated  to  redness,  and  those 
which  are  decomposed  on  heating,  red  ferric  oxide  being  one 
of  the  products.  In  the  first  class  of  compounds,  the  iron 
is  in  the  form  of  a  base  and  may  be  regarded  as  having  partially 
displaced  one  or  more  of  the  other  metallic  bases  as  in  the  micas. 
In  the  second  group  the  iron  forms  part  of  a  complex  acid 
group,  as  in  nontronite  and  other  ferrosilicates  which  are 
analogous  to  clays,  and  on  heating  form  free  silica  and  iron 
oxide. 

Sulphide  of  iron  (FeS2)  occurs  in  four  well-known  forms  in 
clays  :— 

Pyrite  as  nodular  or  kidney-shaped  masses,  which,  on  frac- 
ture, are  seen  to  consist  of  minute  brassy  cubes,  or  as  the 
minute  cubes  separately.  The  brass-like  lustre  is  very  charac- 
teristic, and  pyrite  is  occasionally  mistaken  for  metallic 
gold. 


310  THE  HAW  MATERIALS  FOR  BRICKS 

Marcasite,  as  fibrous  masses  and  twin-rhombic  crystals,  and 
occasionally  as  nodules. 

Mundic,  as  root  or  twig-like  masses  of  great  relative  weight, 
which,  on  fracture,  are  found  to  consist  of  either  pyrite  or 
marcasite. 

Chalcopyrite,  as  nodules  resembling  pyrite,  but  differing  in 
containing  copper  sulphide  as  well  as  iron  sulphide. 

All  forms  of  iron  sulphide  are  objectionable  in  brick  clays,  as 
they  form  black  spots  or  "  splashes  "  of  black  slag  on  the 
surface  of  the  fired  goods.  They  occur  less  frequently  in  the 
red-burning  clays  than  in  the  fire  clay  and  buff-burning  ones, 
the  pyrites  originally  present  in  the  first-named  having,  it  is 
believed,  been  oxidised  previous  to,  or  shortly  after,  its  intro- 
duction into  the  clay. 

Hydrates  of  iron — of  which  limonite  is  the  best  example — 
yield  red-ferric  oxide  on  heating,  and  evolve  water.  The 
proportion  of  water  in  chemical  combination  with  the  iron 
appears  to  vary  greatly,  and  the  formula  2Fe.2O^H.20,  usually 
attributed  to  limonite,  cannot  be  relied  upon.  These  hydrates 
usually  occur  in  nodules  which  may  be  picked  out  of  the  clay- 
especially  when  it  has  been  exposed  to  the  weather — but  they 
also  occur  distributed  throughout  the  clay  mass  in  so  fine  a 
state  of  division  as  to  give  the  raw  clay  a  yellow,  brown,  or 
even  red  colour.  The  proportion  of  combined  water  in 
limonite  and  allied  compounds  appears  to  influence  the  colour  ; 
this  water  may  not  be  in  strict  chemical  combination,  however, 
but  rather  as  a  colloidal  material. 

Ferrous  compounds  are  not  readily  noticeable  in  raw  clays, 
but  in  those  which  have  been  heated  they  form  bluish-grey  or 
black  slags,  to  which  the  well-known  "  blue  Staffordshire 
bricks  "  owe  their  characteristic  colour,  the  ferrous  compounds 
being  formed  in  the  kiln  by  the  action  of  the  reducing  gases 
from  the  fuel. 

Carbonaceous  matter,  derived  from  plants  and  animal  remains, 
occurs  in  many  clays,  and  usually  gives  them  a  greyish  or  brown 
colour.  If  this  impurity  has  become  carbonised  the  clay  may 
be  quite  black,  though  the  total  proportion  of  carbon  may  not 
exceed  5  or  6  per  cent. 

All  carbonaceous  matter  burns  away  when  clays  are  slowly 


IMPURITIES  IN  CLAYS  311 

heated  in  contact  with  air,  so  that  it  has  no  direct  influence  on 
the  colour  of  the  finished  goods.  Indirectly,  it  tends  to  produce 
a  reducing  atmosphere  in  the  kiln,  and  so  may  effect  the 
reduction  of  some  ferric  oxide  and  the  formation  of  the  bluish 
ferrous  silicate. 

When  carbonaceous  matter  is  burned  out  of  a  clay,  the  latter 
becomes  porous  ;  hence,  sawdust,  etc.  is  sometimes  added  to 
a  clay  when  specially  porous  bricks  are  desired. 

Other  impurities  are  occasionally  important  in  clays  to  be 
used  for  special  purposes.  They  usually  reduce  the  heat 
resistance  of  a  clay  (as  titanium  oxide),  or  affect  its  colour 
when  burned  (as  tourmaline),  but  are  not  ordinarily  of  import- 
ance in  brickmaking,  and,  therefore,  need  no  further  mention 
in  the  present  volume.  Further  information  concerning  them 
will  be  found  in  the  author's  "  British  Clays,  Shales  and 
Sands." 

The  following  clays  are  of  sufficient  importance  in  brick- 
making  to  be  described  here  in  addition  to  those  previously 
mentioned  : — 

Agglomerate  clay  is  a  mixture  of  angular  stones  and  fragments 
of  rock  cemented  together  with  a  plastic  clay.  It  is  of  little 
or  no  value. 

Alluvial  clay  collects  in  valleys  in  various  parts  of  the 
country,  and  is  usually  of  very  fine  texture.  In  composition 
and  general  character  it  is  liable  to  vary  very  greatly  within 
small  areas,  and  is,  therefore,  less  trustworthy  than  marine 
deposited  clays.  Alluvial  clay  is  often  rich  in  chalk  and  lime- 
stone dust,  and  is  then  of  insignificant  value. 

The  term  "  alluvial  clay  "  is  often  used  to  distinguish  light 
fine  clays  of  low  plasticity  from  the  compact,  strongly  plastic 
clays,  and  from  the  loams  and  shales. 

Ball  clays  are  not  intentionally  made  into  bricks  unless  the 
clays  are  of  exceptionally  low  quality.  The  better  qualities 
are  too  valuable,  and  are  used  for  pottery  manufacture. 

Brick  earth  is  a  term  used  to  denote  any  material  containing 
clay  which  can  be  made  into  good  bricks.  Some  natural 
products  contain  so  little  clay  that  they  cannot  suitably  be. 
termed  "  clays,"  and  yet  they  can  be  made  into  good  bricks. 
Many  loams  and  true  marls  are  of  this  class,  but  the  term  may 


312  THE  RAW  MATERIALS  FOR  BRICKS 

be  applied  with  accuracy  to  any  clay  or  clay  mixture  which 
can  be  made  into  good,  merchantable  bricks. 

Clunches  are  clays  which  are  mined  and  not  quarried,  and 
may  be  used  for  a  common  brick  shale  or  clay  or  for  a  high- 
class  firebrick.  The  term  has  nothing  to  do  with  their 
composition. 

Conglomerate  days  are  similar  to  agglomerate  clays  (q.v.), 
but  the  rock  particles  are  rounded  instead  of  angular.  They 
are  of  little  or  no  value  for  brickmaking. 

Drift  days  are  commonly  termed  "  Boulder  clays,"  and  form 
the  chief  brickmaking  material  in  the  north  of  England,  where 
they  occur  in  enormous  quantities  as  the  residue.  As  explained 
on  p.  296,  these  clays  make  good  building  bricks  when  suffi- 
ciently free  from  stones,  but  they  are  liable  to  be  spoiled  by 
fragments  of  limestone  in  the  gravels  with  which  they  are 
associated.  With  care  this  difficulty  may  be  overcome,  and 
good,  hard  bricks  produced. 

Fat  days  are  those  which  are  oleaginous  in  consistency  and 
usually  possess  a  high  degree  of  plasticity.  When  used  alone 
they  are  seldom  satisfactory  for  brickmaking,  but  on  the 
addition  of  grog,  sand,  or  very  lean  clay,  they  form  excellent 
bricks. 

Firedays  are  those  which  can  withstand  exposure  to  a 
temperature  rather  higher  than  that  of  molten  steel  without 
showing  signs  of  fusion,  and  are,  consequently,  valuable  for 
furnaces,  kilns,  and  other  structures  where  a  refractory  material 
is  required. 

Fireclays  are  mined  from  pits  sunk  in  the  Coal  Measures,  and 
are  closely  associated  with  all  the  best  known  seams  of  coal. 
The  manufacture  of  firebricks,  retorts,  etc.,  is,  therefore,  largely 
undertaken  by  colliery  proprietors,  or  by  firms  working  in 
connection  with  them.  The  best  fireclays  occur  in  the  Stour- 
bridge,  Yorkshire,  Northumberland  and  Scotch  coalfields,  but 
others  of  almost  equal  quality  are  found  in  other  districts, 
particularly  in  Wales. 

The  chief  characteristics  required  in  a  fireclay  are  resistance 
to  heat  and  to  sudden  changes  of  temperature,  but  other 
requisite  properties  are  resistance  to  the  corrosive  action  of 
slags,  dross  and  metallic  compounds,  and  to  the  abrasive 


FIRECLAYS  313 

action  of  lumps  of  lime,  stone  or  ore  and  of  flue-gases.  As  it 
is  impossible  to  obtain  a  fireclay  which  can  be  used  for  all  the 
purposes  for  which  firebricks,  etc.  are  used,  it  is  necessary  to 
select  certain  clays  for  certain  purposes.  For  this  reason  the 
fireclays  from  different  localities  are  specially  adapted  for 
different  purposes.  The  colour  of  raw  fireclays  is  a  dirty  grey, 
which  becomes  buff  or  stone  colour  on  heating,  the  fired  product 
being  usually  spotted  with  grains  of  ferrous  silicate  due  to  the 
pyrites  in  the  clay  (p.  309). 

Fireclays  are  largely  used  in  the  manufacture  of  crucibles, 
for  which  purpose  they  are  frequently  mixed  with  plumbago, 
which  renders  them  less  sensitive  to  sudden  changes  in  tempera- 
ture and  tends  to  maintain  a  reducing  atmosphere  in  contact 
with  the  contents  of  the  crucibles.  This  is  important  in  some 
metallurgical  operations. 

Allied  to  the  fireclays,  though  not  really  a  clay,  but  a 
siliceous  rock  with  a  low  clay  content,  is  the  material  known  as 
ganister,  which  usually  occurs  in  the  coalfields  below  the  coal 
seams  :  it  is  occasionally  found  nearer  the  surface,  and  in 
such  cases  the  previously  superincumbent  coal  has  probably 
been  removed  by  denudation. 

The  subject  of  fireclays  is  a  very  large  one,  and  the  reader 
desiring  more  information  should  therefore  consult  the  author's 
"  British  Clays,  Shales  and  Sands." 

Fusible  clays  may  be  regarded  as  the  opposite  of  fireclays, 
as  they  undergo  partial  fusion,  and  form  a  hard,  vitrified  mass 
when  heated.  If  such  clays  retain  their  shape  at  a  temperature 
approaching  the  melting  point  of  steel,  they  are  exceedingly 
valuable,  and  under  the  term  "  stoneware  "  are  employed  on 
a  very  extensive  scale  (see  "  Verifiable  Clays  "). 

Engineering,  clinker  and  paving  bricks  are  made  of  some- 
what fusible  clays  which  are  able  to  retain  their  shape  at  a 
high  temperature,  notwithstanding  the  larger  amount  of 
vitrification  which  takes  place.  They  owe  their  strength  to 
the  tenacity  with  which  the  infusible  particles  in  the  clay  are 
bound  together  by  the  more  fusible  constituents. 

Gault  is  a  stiff,  dark-coloured  clay  which  occurs  in  the 
Greensand  formation  in  the  south-eastern  counties,  and  extends 
westward  towards  Midhurst  and  southward  to  Eastbourne. 


314  THE  RAW  MATERIALS  FOR  BRICKS 

Owing  to  the  calcium  carbonate  it  contains,  bricks  made  from 
it  are  white.  Some  gault  bricks  are  reddish  in  colour,  but  white 
ones  may  be  made  by  the  addition  of  chalk.  Wherever  it 
crops  out  near  the  surface  of  the  ground,  gault  clay  is  found 
suitable  for  brickmaking,  the  exceptions  to  this  statement  being 
few,  and  chiefly  in  some  parts  of  Cambridgeshire  where  the 
clay  is  more  unctuous  (see  p.  6). 

Though  largely  used  for  brickmaking — especially  after  the 
addition  of  chalk — gault  clay  cannot  be  regarded  as  a  high-class 
material,  though  for  common  bricks  it  is  excellent. 

Grog  is  simply  clay  which  has  been  burned  in  a  kiln  and  then 
reduced  to  powder.  The  clay  may  be  calcined  specially,  or 
the  grog  may  be  made  by  crushing  broken  bricks.  According 
to  the  nature  of  the  original  clay,  so  will  the  grog  be  refractory 
or  fusible,  soft  or  hard,  buff-coloured  or  red.  The  chief  uses 
of  grog  are  to  increase  the  heat -resistance  of  a  fusible  clay 
and  to  reduce  the  plasticity  and  shrinkage  of  a  clay  which 
would  otherwise  be  difficult  to  work. 

Kaolin  or  china  clay  (the  latter  being  one  kind  of  kaolin)  is 
not  used  for  brickmaking,  though  the  sandy  residues  obtained 
by  washing  are  made  into  bricks  in  some  localities.  The  value 
of  good  kaolin  to  paper  makers  and  others  is  so  great  and 
its  plasticity  is  so  low  that  it  is  not  likely  to  meet  with 
extended  use  in  the  manufacture  of  building  bricks  in  this 
country. 

Laminated  clays  are  those  composed  of  layers  or  flakes  and 
are  troublesome  to  use,  as  they  tend  to  split  along  the  lines  of 
deposition.  When  hard,  they  form  shales  (p.  316)  to  which 
this  objection  is  less  applicable,  as  the  harder  material  can  be 
reduced  to  particles  so  small  that  their  lamination  becomes 
imperceptible. 

Lean  clays  may  be  regarded  as  the  opposite  of  "  fat  "  ones 
(p.  312)  and  have  only  slight  plasticity.  Clays  of  moderate 
leanness  are  the  most  useful  for  brickmaking,  as  they  do  not 
shrink  unduly  in  drying.  Indeed,  many  plastic  clays  which, 
by  themselves,  would  be  useless,  may  be  made  valuable  by 
the  addition  of  sufficient  sand  or  grog  to  convert  them  into 
moderately  lean  clays.  Clays  which  are  so  lean  that  they  do 
not  readily  retain  the  shape  into  which  they  have  been  formed 


VARIOUS  CLAYS  315 

are  of  little  value  unless  some  of  the  sand  or  other  non-plastic 
material  they  contain  can  be  removed  cheaply. 

London  clay  is  one  of  the  best  known  clays,  and  is  at  the  same 
time  one  of  the  most  risky  for  the  manufacture  of  bricks. 
It  occurs  chiefly  around  London  and  occupies  a  very  extensive 
area.  Unfortunately,  it  is  so  sticky  and  contractile  that 
alone  it  is  practically  useless  for  brickmaking,  but  in  localities 
where  it  occurs  in  close  association  with  sand,  it  is  a  valuable 
material  for  this  purpose.  It  is  to  be  regretted  that  in  many 
localities  where  it  would  be  most  useful  the  absence  of  sand 
renders  it  of  no  value. 

The  portions  of  London  clay  which  are  worked  are  mixed 
with  sifted  cinder  dust  (technically  termed  soil)  and  chalk, 
whereby  the  contraction  is  reduced  and  bricks  obtained  which 
have  been  long  famous  for  their  ability  to  resist — as  no  other 
building  material  appears  to  do — the  corrosive  action  of  the 
atmosphere  and  climate  of  the  Metropolis. 

Marine  deposited  clays  are  those  which  were  originally 
deposited  on  an  ocean  bed,  but  now  form  dry  land.  They 
occur  in  many  parts  of  Great  Britain,  that  known  geologically 
as  the  Oxford  clay  being  one  of  the  most  important,  particularly 
near  Peterborough  and  Fletton,  where  some  of  the  largest 
brickworks  in  the  country  are  situated. 

The  Midland  marls  are  Triassic  clays  of  a  friable  texture 
and  moderate  plasticity,  and  are  not  true  marls  (p.  9). 
They  are  well  known  as  the  raw  material  from  which  the 
excellent  terra-cotta  and  red  facing  bricks  in  the  Midlands  are 
made.  These  clays  cover  a  very  extensive  area,  and  are 
largely  used  in  places  so  far  apart  as  Nottingham,  Leicester, 
Shropshire  and  Wrexham. 

Reading  clays  form  the  western  end  of  the  London  basin 
and  are  much  esteemed  for  the  manufacture  of  red  tiles  and 
bricks.  They  are  of  moderate  plasticity  and  not  particularly 
difficult  to  work,  providing  that  they  are  carefully  selected. 

Refractory  clays  have  been  sufficiently  described  under 
"  Fire  clays  "  (p.  312). 

Rock  clays  are  indurated  clay  deposits  which  have  attained 
a  hardness  and  form  corresponding  to  that  of  rocks,  owing 
to  the  pressure  of  neighbouring  strata  and  the  interpenetration 


316  THE  RAW  MATERIALS  FOR  BRICKS 

of  cementitious  solutions.  Shales,  slates  and  fireclays  are 
typical  rock  clays. 

Sandy  clays  are,  .as  their  name  implies,  mixtures  of  clay  and 
sand  which  contain  so  large  a  proportion  of  the  latter  substance 
as  to  have  a  sandy  nature.  As  "  loams  "  they  are  valuable, 
but  if  too  rich  in  sand  ("  clayey  sands  ")  they  are  useless  for 
brickmaking,  as  they  do  not  possess  sufficient  cohesion. 

Shales  are  laminated,  indurated  clays  which  must  be  crushed 
to  powder  and  kneaded  with  water  before  they  become  plastic. 
They  occur  in  many  localities — usually  at  some  depth  below 
the  surface  of  the  ground — and  their  composition  varies  greatly. 
Some  shales  form  buff,  and  others  red  bricks,  and  a  few  shales 
contain  so  much  fusible  matter  that  blue  engineering  bricks 
can  be  made  from  them. 

Most  shales  contain  only  a  small  percentage  of  carbonaceous 
matter,  but  others  contain  so  much  shale  oil  that  they  are  very 
difficult  to  use  for  brickmaking,  and  a  specially  designed  kiln 
is  usually  necessary  for  them. 

The  shales  chiefly  used  for  brickmaking  occur  in  the  Oxford 
clay,  Lias,  Wealden  and  Coal  Measure  formations.  The  loca- 
tion and  extent  of  these  can  be  seen  in  any  good  geological 
atlas  (see  also  p.  8). 

Silt  is  an  extremely  fine  sand  which  usually  contains  sufficient 
clay  to  form  a  plastic  mass.  It  is  chiefly  found  on  the  sites  of 
ancient  rivers  and  in  low-lying  districts,  and  is  found  in  large 
quantities  in  the  eastern  counties.  The  silt  found  near  Hull 
(and  termed  warp)  has  been  largely  used  for  brickmaking  with 
considerable  success,  but  it  needs  special  knowledge  and  skill 
to  obtain  good  results,  and  would  probably  prove  disastrous 
to  anyone  not  acquainted  with  its  peculiar  characteristics. 

Slates  are  very  hard  clays,  which  have  undergone  a  partial 
recrystallisation.  They  are  not  extensively  used  for  brick- 
making,  though  the  accumulation  of  rubbish  in  slate  quarries 
has  caused  many  attempts  to  be  made  to  convert  this  material 
into  bricks.  The  material  is  not  well  adapted  to  this  purpose 
and  could  only  be  used  commercially  in  localities  where  ordinary 
bricks  were  unobtainable. 

Soil  is  the  uppermost  layer  of  earthly  material,  and  is  the 
"home"  of  plants.  It  does  not  consist  entirely  of  mineral 


VARIOUS  CLAYS  317 

matter,  but  usually  contains  a  considerable  proportion  of 
materials  derived  from  the  decay  of  plants  as  well  as  of  animal 
excreta  and  artificial  fertilisers.  On  this  account  it  is  not 
usually  suitable  for  inclusion  in  the  material  employed  for 
brickmaking,  and  should,  in  most  cases,  be  kept  separate. 
The  soil  overlying  a  useful  deposit  of  clay  is  commonly  termed 
the  overburden  or  callow,  and  is  generally  removed  before  the 
underlying  clay  is  dug. 

Strong  days  are  highly  plastic  and  shrink,  crack  and  warp 
extensively  when  made  into  bricks  and  dried.  They  are 
objectionable  to  most  brickmakers,  who  commonly  term 
them  foul  days.  If  mixed  with  sand  or  grog,  they  usually 
make  good  brick  earths,  but  the  difficulty  and  cost  of  obtaining 
a  satisfactory  mixture  often  prohibits  their  use. 

Surface  days  are,  strictly,  any  clays  which  occur  near  the 
surface  of  the  ground  and  immediately  below  the  soil.  The 
term  is,  however,  used  colloquially  by  many  brickmakers  to 
indicate  a  red-burning  clay  as  distinct  from  a  buff-burning 
material  obtained  from  a  greater  depth.  This  use  of  the 
term  has  led  to  much  confusion  in  the  past  and  should  be 
avoided. 

Tender  days  are  those  which  crack  or  warp  when  made  into 
bricks.  They  are  usually  characterised  by  a  high  shrinkage 
and  great  plasticity.  The  tenderness  may  be  reduced  by  the 
addition  of  sand  or  chalk,  but  some  tender  clays  are  so  peculiarly 
constituted  that  the  addition  of  non-plastic  materials  makes 
them  so  weak  and  friable  as  to  be  useless.  Tender  clays  are 
the  ruin  of  brickmakers  who  do  not  know  how  to  deal  with 
them,  as  they  require  expert  knowledge  before  their  use  can 
be  satisfactory. 

Till  day  is  the  plastic  clay  forming  the  lower  portion  of  the 
boulder  clay  (p.  294),  and  is  valued  for  brickmaking  on  account 
of  its  freedom  from  stones  and  gravel.  The  term  is,  however, 
applied  in  a  loose  manner  to  boulder  clay  generally,  in  which 
case  its  special  significance  is  lost. 

Verifiable  days  are  used  in  brickmaking  for  the  production 
of  bricks  of  great  strength  and  imperviousness  to  water  and 
other  fluids.  Vitrification  is  the  state  in  which  a  portion  of 
the  clay  fuses  and  binds  the  remaining  particles  firmly  together, 


318  THE  RAW  MATERIALS  FOR  BRICKS 

the  fused  portion  filling  some  or  all  of  the  pores  previously 
existing  in  the  material.  The  extent  to  which  vitrification 
occurs  depends  on  the  nature  of  the  fusible  material  present 
and  on  the  temperature  and  duration  of  the  heating  ;  it  is 
said  to  be  complete  when  the  whole  of  the  pores  have  been 
completely  filled  with  fused  material.  Nearly  all  clays  can 
be  vitrified  if  a  sufficiently  high  temperature  is  reached,  but- 
many  of  them  lose  their  shape  before  vitrification  is  complete. 
The  commercial  value  of  vitrifiable  clays,  therefore,  depends 
on  a  considerable  time  elapsing  between  the  production  of  a 
sufficient  amount  of  fused  material  to  close  a  sufficient  number 
of  pores  and  the  commencing  of  a  noticeable  loss  of  shape. 
Clays  which  begin  to  vitrify  and  then  to  lose  shape  almost 
instantaneously  or  without  any  appreciable  rise  in  temperature 
are  useless  commercially  as  vitrifiable  clays.  The  time  of 
heating  or  the  rising  temperature  required  between  these  two 
changes  is  termed  the  "  range  of  vitrification,"  and  it  is  an 
important  factor  in  deciding  the  value  of  a  clay.  Clays  in 
which  the  more  readily  fusible  portion  is  rich  in  potash,  soda  or 
lime,  usually  have  a  short  range  of  vitrification,  and  are 
therefore  less  valuable  than  clays  in  which  the  chief  flux  is 
magnesia,  and  therefore  possess  a  longer  range  of  vitrification. 

The  vitrifiable  clays  chiefly  used  in  brickmaking  are  found 
in  the  Midlands,  where  they  form  the  buff  paving  bricks  of 
the  Little  Bytham  district  and  the  famous  blue  bricks  of 
Staffordshire,  but  equally  good  bricks  can  be  obtained  from 
selected  seams  in  most  of  the  coalfields.  The  purer  stoneware 
clays  of  Dorset,  Devon  and  Cheshire  are  too  valuable  for  this 
purpose,  except  where  bricks  impervious  to  strong  acids  are 
required  for  use  in  chemical  works  and  for  other  special 
purposes. 

Yellow  clays,  i.e.,  those  which  are  yellow  when  freshly  dug, 
are  usually  strong  clays  (p.  317)  and  difficult  to  work  without 
the  addition  of  sand,  but  the  yellow  colour  gives  so  little 
indication  of  their  nature  that  further  tests  are  necessary 
before  their  value  can  be  ascertained. 


CHAPTER  XII 

METHODS   OF   BRICKMAKING 

FROM  what  has  been  stated  on  the  foregoing  pages,  the 
reader  will  easily  perceive  that  the  variety  of  clays  available 
and  their  complex  nature  make  the  manufacture  of  bricks  a 
work  requiring  more  skill  than  is  generally  supposed. 

Capital  required. — Before  commencing  the  manufacture  of 
bricks,  and  allied  articles,  there  are  two  essentials  which  must 
be  considered,  viz.,  capital  and  knowledge.  Without  sufficient 
capital  the  risks  of  failure  are  very  great,  because  the  manu- 
facture of  bricks  is  subject  to  many  vicissitudes  which  cannot 
be  overcome  without  ample  financial  backing. 

The  amount  of  capital  needed  depends  greatly  on  the 
locality  of  the  works  ;  in  an  undeveloped  area,  where  common 
bricks  will  be  all  that  is  desired,  the  money  needed  will  be 
small — a  few  hundred  pounds — but  for  a  plant  with  an  annual 
output  of  a  million  or  more  bricks,  the  capital  required  will  be 
much  greater.  In  the  case  of  an  "  average  "  works  producing 
20,000  bricks  per  day,  it  may  easily  reach  to  £10,000,  and  many 
works  with  this  output  have  a  capital  more  than  four  times 
this  amount.  It  is  only  fair  to  add,  however,  that  the  capital 
invested  in  a  large  number  of  brickworks  is  to  .a  considerable 
extent  "  lost,"  it  having  been  spent  in  continuous  attempts 
to  improve  the  product  by  methods  which  a  man  with  sufficient 
technical  and  scientific  knowledge  could  have  predicted  would 
be  futile. 

The  cases  where  men  have  made  fortunes  out  of  brickmaking 
become  fewer  each  year,  and  an  investigation  of  most  of  them 
will  show  that  whilst  these  brick  manufacturers  may  not  have 
had  much  cash  of  their  own,  yet  the  credit  they  have  been 
able  to  obtain  and  other  facilities  they  have  possessed  have 
had  the  same  effect  as  the  possession  of  considerable  capital. 

A  man  who  would  start  a  brickworks  in  the  United  Kingdom 


320  METHODS  OF  BRICKMAKING 

at  the  present  time  without  ample  capital  in  some  form  or 
other  would  be  almost  certain  to  fail.  With  sufficient  capital, 
however,  there,  are  excellent  prospects  in  a  number  of 
localities,  particularly  in  the  Midlands. 

Technical  Knowledge  Needed. — Most  brickmakers  who  started 
in  business  forty  years  or  more  ago  did  so  under  conditions 
very  different  from  those  at  the  present  time.  The  use  of 
machinery  for  this  purpose  was  practically  unknown,  and  the 
profits  obtained  were  much  higher  than  at  the  present  time. 
With  little  or  no  literature  on  the  subject  the  brickmakers  of 
half  a  century  ago  were  only  able  to  use  a  very  limited  number 
of  clay  deposits,  and  those  of  a  peculiarly  favourable  character. 

The  introduction  of  machinery  effected  a  complete  revolution 
in  the  technical  equipment  required,  it  increased  enormously 
the  number  of  clays  and  earths  which  could  be  made  into 
bricks,  and  created  an  impression — as  false  as  it  is  widespread 
— that,  so  long  as  an  earthy  material  feels  plastic  when  mixed 
with  a  little  water,  good  bricks  can  be  made  from  it  by  any 
man  possessing  the  necessary  plant. 

This  belief  has  caused  the  loss  of  hundreds  of  thousands  of 
pounds,  for  the  manufacture  of  bricks  is  an  industry  requiring 
much  technical  knowledge,  and  it  is  quite  a  mistake  to  imagine 
— as  many  engineers  do — that  all  that  is  requisite  are  a  few 
moulds  or  a  machine,  a  kiln,  and  a  few  labourers.  With  good 
fortune  the  clay  available  may  happen  to  be  one  which  is 
easily  worked  in  the  manner  which  appeals  most  to  the  pros- 
pective brickmaker,  but  the  enormous  number  of  derelict 
works  and  of  machines  which  fail  to  find  purchasers  prove 
that  those  who  regard  brickmaking  merely  as  an  elementary 
branch  of  engineering  run  risks  of  the  most  serious  financial 
character. 

In  the  author's  experience  as  an  expert  adviser  for  many 
years  past,  he  has  found  numberless  cases  of  firms  who  were 
entirely  mistaken  as  to  the  correct  methods  of  working  the 
particular  clay  available  to  them,  and  who,  in  consequence, 
have  lost  enormous  sums  of  money  which  might  have  been 
saved  had  they  obtained  reliable  and  independent  expert 
advice  before  purchasing  plant  or  ordering  the  erection  of 
kilns. 


TECHNICAL  KNOWLEDGE  NEEDED  321 

In  order  that  a  brickworks  may  be  a  commercial  success,  it 
is  necessary  that  only  a  reasonable  amount  of  skill  shall  be 
needed  in  the  production  of  bricks,  and  it  is  precisely  because 
of  failure  to  realise  this  fact  that  so  many  brickworks  prove  to 
be  failures.  Given  ample  time  and  labour,  a  few  good 
merchantable  bricks  can  be  made  with  almost  any  machinery 
on  the  market,  but  this  is  far  different  from  the  conditions 
which  must  prevail  in  a  brickworks  working  commercially. 
The  result  is  that  many  prospective  brick  manufacturers  are 
entirely  misled  by  the  sample  bricks  made  from  their  clay 
by  enterprising  firms  of  machinery  makers  and  kiln  builders, 
and  find,  when  too  late,  that  they  have  prosecuted  their 
enquiries  in  the  wrong  direction. 

Almost  equally  serious  and  erroneous  are  so-called  "  tests  " 
made  by  "  practical  men  "  on  small  quantities  of  clay.  These 
"  match  box  tests  "  (so  called  because  a  match  box  is  frequently 
used  in  place  of  a  proper  mould)  may  or  may  not  yield  results 
worth  the  labour  expended  on  them,  but  in  any  case  it  is  exceed- 
ingly foolish  to  erect  a  works  on  so  slight  a  result.  The  pro- 
duction of  even  1,000  bricks  by  a  friendly  brickmaker  is  by  no 
means  conclusive  evidence  of  the  nature  of  the  clay,  as  there 
are  many  other  matters  to  be  considered  in  manufacturing, 
of  which  these  tests  give  no  indication. 

Finally,  it  is  almost  useless  to  have  an  analysis  made  of  the 
clay.  Certain  chemical  and  physical  tests  are  essential,  but 
a  chemical  analysis  as  conducted  by  a  works  chemist  or  a  public 
analyst  is  of  scarcely  any  value,  as  it  does  not  give  the  informa- 
tion required. 

The  only  satisfactory  method  of  ascertaining  whether  a 
given  clay  is  likely  to  be  suitable  for  the  manufacture  of  bricks 
is  to  have  it  examined  by  an  expert  in  clay  testing,  who  is 
financially  independent  of  all  machinery  and  kiln  constructing 
firms,  and  who  is  known  not  to  accept  secret  commissions, 
rebates  or  other  inducements  which  will  bias  his  recommenda- 
tions. The  fees  charged  by  such  a  man  will  be  saved  in  the 
avoidance  of  unnecessary  machinery  and  in  the  absence  of 
those  annoying  "  extras "  by  which  the  original  estimate 
of  costs  is  usually  increased  so  largely,  but  which  are  almost 
unavoidable  in  the  absence  of  such  skilled  technical  supervision. 

C.  Y 


322  METHODS  OF  BRICKMAKING 

The  man  who  wishes  to  build  a  house  employs,  if  he  is  wise, 
an  architect  to  advise  and  assist  him  in  its  design  and  erection, 
and  to  check  the  tendency  to  extravagance  on  the  part  of 
both  owner  and  builder.  The  man  who  is  ill  is  most  likely 
to  recover  if  he  seeks  the  aid  of  a  medical  man,  for  an  able 
physician,  who  has  made  a  special  study  of  the  subject,  wastes 
no  time  in  effecting  a  cure.  "  A  man  who  is  his  own  lawyer 
has  a  fool  for  a  client  "  is  a  proverb  famous  alike  for  its  general 
applicability  and  its  truthfulness.  Yet  an  error  in  the  selection 
of  brick  machinery  or  kilns,  or  in  the  valuation  of  a  clay  pro- 
perty, may  easily  involve  the  loss  of  several  thousands  of 
pounds,  which  the  employment  of  a  reliable  expert  in  clay 
working  would  have  saved. 

Many  instances  might  be  quoted  to  illustrate  this,  but  the 
following  recent  one,  from  the  author's  personal  experience, 
must  suffice.  A  certain  landowner  found  a  shale  on  his  land, 
which  he  believed  to  be  valuable,  especially  as  the  neighbour- 
hood was  one  in  which  there  was  an  increasing  demand  for 
bricks.  A  company  was  formed,  machinery  purchased,  kilns 
erected  and  work  begun.  But,  alas  !  the  output  was  less  than 
half  that  anticipated,  and  the  costs  were  slightly  higher  than 
the  selling  price  of  bricks  in  the  neighbourhood  !  Many  weary 
months  of  working  failed  to  improve  the  conditions  and  eventu- 
ally an  expert  was  called  in  to  report  upon  the  whole  works. 
He  found,  as  was  only  to  be  expected,  that  the  machinery 
installed  had  been  seriously  overrated  for  the  particular 
material  to  be  treated,  though  the  output  promised  could 
readily  have  been  obtained  with  a  different  clay.  He  learned 
that  various  firms  had  tendered  for  the  supply  of  machinery, 
and  that  the  accepted  tender  was  for  the  machinery  not 
best  fitted  for  the  work.  The  kilns  had  been  purchased  in  an 
indirect  manner,  the  ordinary  price  for  eight  kilns  had  been  paid 
for  the  five  kilns  erected  (the  balance  representing  a  commis- 
sion to  the  firm  introducing  the  kiln  builders,  and  a  "little 
extra  "  for  the  builders  because  there  was  no  competition). 
Reconstruction  of  the  works  was  imperative,  but  was  rendered 
difficult  because  the  shareholders,  having  been  "  once  bitten  " 
were  "  twice  shy."  Eventually,  however,  the  rearrangement 
of  the  plant  was  completed,  and  it  has  been  working  satisfac- 


TECHNICAL  KNOWLEDGE  NEEDED  323 

torily  ever  since.  Had  the  present  method  of  working  been 
installed  in  the  first  instance — as  would  certainly  have  been 
the  case  had  the  expert  been  consulted  at  a  sufficiently  early 
stage — the  total  amount  saved  would  have  been  £9,000  plus 
the  losses  due  to  working  with  unsuitable  machinery. 

This  is  not  an  isolated  example,  but  is  typical  of  many  in 
various  parts  of  the  United  Kingdom.  It  is  not  fair  to  blame 
the  firm  who  supplied  the  machinery  and  kilns  ;  their  first 
business  was  to  sell  their  own  goods  and  to  make  out  the  best 
case  for  these.  To  have  recommended  the  inclusion  of  plant 
they  did  not  make  was  no  part  of  their  work,  hence  their  advice 
was  necessarily  and  unavoidably  biassed,  whereas  an  expert — 
expressly  chosen  because  of  his  independence  and  freedom 
from  bias  of  this  kind — would  have  specified  the  plant  and  kilns 
most  suitable  for  that  particular  material. 

The  following  description  of  methods  of  manufacturing 
bricks  is  only  intended  to  outline  the  most  important  processes, 
and  the  reader  who  wishes  for  further  information  and  for 
more  illustrations  of  the  machinery,  etc.  employed,  should 
consult  the  author's  "  Modern  Brickmaking  "  •  (Scott, 
Greenwood  &  Son),  the  author's  "  Clay  workers'  Handbook  " 
(Griffin  &  Co.)  or  "  Bricks  and  Tiles,"  by  Dobson  and  Searle 
(Crosby,  Lockwood  &  Co.). 

Mining  and  Quarrying. — The  first  operation  in  the  manufac- 
ture of  bricks  is  the  mining  or  quarrying  of  the  clay  or  brick 
earth  and  its  delivery  to  the  machinery  which  prepares  it 
for  use.  A  description  of  the  various  methods  of  working  in 
clay  pits  and  mines  would  require  a  volume  to  itself,  and  it 
must  here  suffice  to  state  that  in  mining  the  ordinary  colliery 
methods  are  employed,  and  that  in  quarrying  the  clay  is  usually 
obtained  by  means  of  picks  and  shovels,  the  excavation  being 
carried  out  in  a  series  of  shelves  or  ledges.  Very  hard  materials 
are  loosened  by  blasting  with  gelignite  or  other  "  safety  " 
explosives . 

Steam  navvies  and  ditch  cutters  are  used  where  the  material 
is  sufficiently  soft  and  uniform,  but  their  use  is  impracticable 
in  many  brickyards  on  account  of  the  need  for  selecting  certain 
portions  and  discarding  others  from  the  quarry  face. 

Selection  of  suitable  materials  and  rejection  of  unsuitable 

Y  2 


324  METHODS  OF  BRICKMAKING 

ones  will  make  all  the  difference  between  good  and  bad  bricks, 
and  in  most  quarries  too  much  care  cannot  be  taken  to  keep 
detrimental  material  away  from  the  useful  clay.  It  is,  gener- 
ally speaking,  foolish  to  mix  the  materials  indiscriminately 
together  and  expect  good  bricks  to  be  made,  though  this  is 
done  successfully  in  some  yards  where  the  conditions  of 
deposition  of  the  various  materials  has  been  peculiar.  The 
wise  plan  is  to  keep  various  materials  separate,  loading  them 
into  separate  waggons  and  then  mixing  them  in  the  desired 
proportions  in  the  machines.  Such  treatment  secures  a  more 
uniform  product  than  is  possible  when  they  are  mixed  in  the  pit. 

At  one  time,  wheelbarrows  were  used  to  convey  the  clay 
to  the  machines  ;  these  are  now  used  in  some  small  works, 
but  in  the  larger  ones  waggons  are  preferred.  These  waggons 
have  a  capacity  of  5  to  15  cwts.  of  clay,  the  smaller  ones  being 
used  on  steep  inclines  and  the  larger  ones  for  general  work. 
The  waggons  travel  on  rails,  turntables  or  "  points  "  being  used 
at  junctions,  so  as  to  provide  a  good  surface  on  which  the  wheels 
may  rotate.  These  rails  are  of  narrow  gauge  and  are  usually 
of  a  semi-portable  character,  so  that  they  may  be  extended 
rapidly  to  convenient  parts  of  the  clay  pit. 

The  loaded  waggons  are  pushed  by  one  or  more  men  from 
the  working  face  in  the  pit  until  they  reach  the  hauling 
mechanism,  or  ponies  may  be  employed  to  do  the  whole  of  the 
haulage  if  the  clay  hole  is  not  very  deep.  Usually,  the  best 
method  of  haulage  consists  in  the  use  of  an  endless  rope  or 
chain  to  which  the  waggons  are  attached  by  some  simple 
form  of  clip.  In  some  works  the  use  of  a  simple  haulage  rope 
or  a  main  and  tail  system,  such  as  is  used  in  collieries,  is 
preferred,  the  arrangement  most  suitable  in  any  particular  case 
depending  on  the  length  of  haulage,  the  inclines  to  be  nego- 
tiated, the  changes  in  direction  of  travel,  and  other  purely 
local  conditions. 

Overhead  ropeways  are  by  far  the  most  economical  method 
of  conveying  clay  to  the  works  where  the  distance  and 
quantity  are  large,  but  most  brickworks  are  too  small  for  an 
overhead  ropeway  to  be  used. 

Many  brick  manufacturers  spend  far  more  in  haulage  costs 
than  is  really  necessary,  because  of  the  directions  in  which 


MINING  AND  QUARRYING  325 

the  rails  have  been  laid,  but  the  reduction  of  these  costs  is 
too  complex  a  problem  for  a  solution  to  be  attempted  here. 
Suffice  it  to  say  that  the  shorter  and  more  direct  the  course  along 
which  the  waggons  travel — providing  the  inclination  is  not  too 
steep — and  the  nearer  the  further  end  of  the  hauling  plant  is  to 
the  working  face,  the  cheaper  will  be  the  cost  of  transportation. 

As  soon  as  the  waggons  with  their  load  of  clay,  sand  or  other 
material  reach  the  near  or  machine  end  of  their  journey,  the 
second  stage  of  the  manufacture  is  reached. 

Clay  Preparation. — The  conversion  of  the  freshly-mined  or 
quarried  material  into  one  which  is  suitable  for  the  direct 
production  of  bricks  is  by  no  means  a  simple  task.  Clays  and 
brick-earths  vary  so  much  in  composition  and  physical  nature 
that  a  treatment  which  is  ample  for  one  may  be  quite  insuffi- 
cient or  even  unsuitable  in  character  for  another,  and  any 
short  description  of  the  processes  of  preparation  must  neces- 
sarily be  merely  indicative  and  incomplete. 

The  object  of  all  preparation  processes  is  the  production  of 
a  plastic  paste  of  such  a  consistency  that  it  may  readily  be 
formed  into  bricks  or  other  articles  of  any  desired  size  and 
shape.1  In  order  that  this  object  may  be  attained,  the  material 
may  require  to  be  crushed  to  powder  and  then  kneaded  or 
mixed  with  a  suitable  quantity  of  water,  the  latter  treatment 
being  carried  out  in  such  a  manner  as  will  secure  a  paste  of 
as  uniform  a  composition  and  texture  as  possible.  Insufficient 
or  careless  treatment  in  the  preparation  of  the  paste  is  one  of 
the  most  frequent  causes  of  defective  goods. 

Some  clays  occur  naturally  as  a  paste,  which  only  requires 
to  be  kneaded  to  make  it  uniform  and  of  the  desired  consis- 
tency ;  these  are  the  easiest  clays  to  manipulate,  but  the 
quantity  available  is  limited.  Other  clays  require  the  addition 
of  water  arid  possibly  of  sand,  chalk,  cinder  dust  or  other 
non-plastic  materials,  and  some  of  them  require  to  be  crushed 
or  pressed  into  thin  sheets  before  being  kneaded  ;  unless  this 
is  done  the  water  added  will  not  mix  properly,  and  they  cannot 
be  made  into  a  suitable  paste. 

1  The  only  exception  to  this  is  in  what  is  known  as  the  "  dry  dust,"  or  "  semi- 
dry  "  process,  in  which  the  finely  powdered  material  is  compressed  in  powerful 
presses  to  the  desired  shape.  This  method  of  manufacture  is  described  later. 


326  METHODS  OF  BRICKMAKING 

Marls,  loams  and  friable  brick-earths  require  a  much  larger 
proportion  of  water  and  a  more  thorough  kneading  before  a 
uniform  paste  is  produced.  Some  of  these  materials  also 
require  to  be  crushed  or  ground  to  powder  and  sifted  before 
being  mixed  with  water.  If  this  grinding  is  omitted  the 
"  paste  "  will  be  so  irregular  in  texture  as  to  be  quite  useless 
for  brickmaking. 

Shales,  also  clays  containing  stones  or  pieces  of  hard  material, 
must  always  be  ground  to  powder  and  sifted — the  coarse 
particles  being  returned  to  the  mill  and  reground — before 
they  can  be  kneaded  with  water  to  form  a  suitable  paste.  If 
the  clay  is  naturally  of  a  pasty  nature,  with  small  stones  or 
gravel  embedded  in  it,  the  preliminary  crushing  is  sometimes 
extremely  difficult,  the  unctuous  nature  of  the  clay  making  it 
almost  impossible  to  crush  the  other  ingredients  sufficiently 
small. 

During  the  past  few  years  many  clays  of  this  character  have 
been  made  available  by  means  of  a  day  cleaner,  which  consists 
of  a  cylinder  with  a  series  of  small  apertures  through  which 
the  soft,  plastic  material  and  the  finer  grains  of  gravel,  sand, 
etc.,  are  forced,  whilst  the  coarser  particles  are  retained  because 
they  are  too  large  to  pass  through  the  apertures.  Clay-cleaning 
machines  are  of  several  types,  but  the  separating  principle  just 
mentioned  is  their  chief  feature. 

Clays  which  contain  sand  as  well  as  stones  or  gravel  cannot 
be  sufficiently  purified  in  this  manner,  but  must  be  washed  by 
stirring  the  material  with  water  in  a  large  trough  or  shallow 
well,  the  solid  portion  being  broken  up  by  means  of  rotary 
beaters  or  "  hurdles  "  until  a  cream  or  slurry  is  formed.  This 
slurry  is  allowed  to  remain  stationary  for  a  few  moments, 
during  which  much  of  the  sand  settles  out,  and  the  fluid  is 
then  run  off  to  settling  pits,  where,  after  some  time,  the  clear 
supernatant  water  is  cautiously  run  off,  and  the  clay  paste 
remaining  in  the  pits  is  allowed  to  stiffen.  When  sufficiently 
dry  it  is  then  taken  to  the  mixing  machines  in  order  to  render 
its  texture  as  uniform  as  possible.  No  washing,  however 
perfect,  can  completely  remove  the  impurities  from  clay,  but 
the  treatment  just  described  is  extensively  used  by  bookmakers 
in  the  southern  counties,  particularly  in  those  districts  where 


PREPARATION    OF   THE   CLAY  327 

chalk  is  mixed  with  the  clay.  In  such  a  case  the  chalk  is 
made  into  a  slurry  in  a  separate  wash-mill,  and  this  slurry  is 
then  run  into  the  clay-mill  and  afterwards  to  the  settling  pits. 
Before  any  clay  or  brick-earth  is  crushed,  however,  it  should 
be  examined  in  order  to  ascertain  whether  the  same  effect  and 
other  improvements  in  its  nature  cannot  be  obtained  by 
exposure  to  the  weather.  Many  hard  clays  and  earths  are 
reduced  to  a  comparatively  soft  material  when  exposed  to 
frost,  and  not  a  few  of  them  fall  almost  to  powder  if  merely 
exposed  for  a  few  days  to  the  action  of  the  atmosphere.  This 
exposure  (or  weathering)  not  only  facilitates  the  crushing  and 
mixing  of  the  clay,  but  it  frequently  brings  about  chemical 
and  physical  changes  of  the  greatest  importance  to  the  brick 
manufacturer.  The  precise  nature  of  some  of  these  changes 
is  obscure,  but  it  appears  to  be  a  kind  of  oxidation  combined 
with  the  production  of  internal  stresses  and  strains  which 
cause  the  particles  of  clay  to  separate  from  each  other  and  to 
form  a  loosely  coherent  mass.  Weathering  also  effects  the 
purification  of  some  clays  by  causing  the  solution  of  some  of 
the  impurities,  and,  as  the  water  evaporates,  some  of  these 
are  carried  to  the  surface  and  form  concretions  or  a  scum  which 
can  be  picked  or  scraped  from  the  surface. 

The  crushing  machinery  used  in  brickmaking  is  of  three  chief 
types,  consisting  of  : — 

(a)  One  or  more  pairs  of  horizontal  rolls,  each  about  18  inches 
diameter  and  24  inches  wide,  placed  side  by  side  in  such  a 
manner  that  the  raw  material  falls  on  them  and  is  crushed 
to  the  desired  fineness  by  passing  between  them.  Ordinarily 
these  rolls  should  not  be  further  apart  than  the  thickness 
of  a  penny,  but  some  clays  require  them  to  be  set  almost  in 
contact.  The  rolls  tend  to  spring  apart  slightly  in  use,  and 
as  they  become  worn  they  permit  larger  pieces  to  pass  between 
them,  so  that  they  cannot  be  relied  upon  to  reduce  the  material 
to  particles  much  less  than  J  inch  in  thickness,  though  the 
greater  part  will  be  much  smaller.  Where  the  clay  contains 
stones  or  hard  lumps  the  output  of  the  machine  can  be  increased 
by  the  use  of  two,  three  or  even  four  pairs  of  rolls,  placed 
one  above  the  other,  the  rolls  in  each  pair  being  set  rather 
further  apart  than  in  the  pair  below.  Thus,  the  lowest  rolls 


328 


METHODS    OF   BRICKMAKING. 


may  be  -fy  inch  apart,  the  middle  pair  J  inch  apart,  and  the 
ones  in  the  upper  pair  may  be  an  inch  or  more  from  each  other. 
If  the  clay  is  exceptionally  tenacious  an  additional  pair  of 
spiked,  grooved  or  studded  rolls  may  be  necessary,  as  lumps 
of  such  material  would  slip  on  smooth  rolls  and  would  not  be 
crushed.  Such  rolls  are  termed  kibblers  or  wolves. 

(b)  The  second  type  of  crushing  machine  consists  of  a  pair 
of  rollers  each  about  5  feet  in  diameter  and  9  inches  wide, 


FIG.  96.— Crushing  Rolls. 

fixed  at  opposite  ends  of  a  single  horizontal  shaft  and  rotating 
on  a  pan  or  bed  in  such  a  manner  that  any  material  on  the 
latter  is  crushed  as  the  rollers  pass  over  it.  Such  an  arrange- 
ment is  termed  an  edge-runner  grinding  mill,  and  it  is  specially 
adapted  for  crushing  dry  or  slightly  moist  rocks,  hard  clays 
and  shales.  Two  patterns  of  edge-runner  mills  are  in  use  ; 
in  one,  the  bed  is  fixed  and  the  rollers  run  over  it,  dragging 
behind  them  scrapers  which  remove  the  crushed  material 
and  conduct  it  to  an  elevator,  whilst  in  the  second  form  of 


PREPARATION   OF   THE   CLAY  329 

mill  the  rollers  are  carried  loosely  in  a  framework  and  merely 
rotate  by  friction,  whilst  the  pan  or  bed  is  rotated  rapidly 
by  mechanical  power.  This  pan  is  perforated  or  provided 
with  slots  over  a  considerable  part  of  its  base,  and  as  the 
material  is  crushed  between  the  rolls  and  the  pan,  it  passes 
through  these  perforations  and  falls  into  a  well,  fitted  with  an 
elevator. 

The  size  and  weight  of  the  rollers,  the  speed  at  which  the 
rollers  or  pan  is  driven,  together  with  the  size  of  the  apertures 
in  the  base,  determine  the  output  of  such  mills,  the  nature 
of  the  material  to  be  crushed  being  also  an  important  factor. 

Edge-runner  mills  of  these  patterns  are  not  suitable  for  very 
pasty    materials,    though    if    the    slots    are    sufficiently    large 
(each  about  3  inches  by  J  inch)  they  form  admirable  preliminary 
crushers  where  very  adhesive  clays 
are  worked.      The    most   suitable 
clay  for  crushing  in  an  edge-runner 
mill  is  a  friable  marl,  loam  or  soft 
shale  which   is   not  moist  enough 
to  adhere  too  closely   to  the  pan 
or  rolls.     Some  shales  and  fireclays 
are   so    dry    that    a    considerable 
amount  of  water    may    be  added 

to  them     during     grinding     with-         FIG    97.— Edge-runner 

,,.  '    .&  ,.,  ,  Grinding  Mill, 

out    their    becoming    perceptibly 

plastic  or  adhesive.  The  use  of  this  water  prevents  the  loss 
of  the  finest  particles  of  clay  which  would  otherwise  escape 
in  the  form  of  fine  dust. 

It  is  seldom  possible  to  have  the  perforations  in  the  pan  of 
an  edge-runner  mill  less  than  J  inch  diameter,  and  smaller 
ones  so  rapidly  become  larger  with  wear  and  tear  that  |-inch 
holes  are  rightly  regarded  as  the  least  size  practicable.  Much 
of  the  crushed  material  is  far  finer  than  this,  and  some  users 
of  bricks  demand  such  a  texture  that  the  whole  of  the  material 
must  be  passed  through  a  sieve  with  ten,  twelve,  fourteen  or 
even  eighteen  holes  per  linear  inch.  It  is  therefore  customary 
to  lift  the  powder  which  issues  from  the  edge-runner  mill  to 
a  screen  or  riddle  or  suitable  mesh,  which  is  fixed  on  an  upper 
floor  well  above  the  grinding  plant.  The  fine  material  which 


330  METHODS    OF   BRICKMAKING 

has  passed  through  the  screen  falls  on  to  a  floor  or  into  a  receiv- 
ing hopper  and  the  coarse  material  or  tailings  is  returned  to 
the  mill  for  further  treatment. 

In  order  to  obtain  a  uniform  paste  it  is  necessary  to  mix  the 
clay  or  brickmaking  material  with  water  in  a  very  thorough 
manner.  Hence,  the  general  necessity  for  first  crushing  the 
clay  into  thin  sheets  or  of  grinding  it  to  powder  as  described. 
The  water  and  material  so  prepared  (if  necessary)  may  then 
be  kneaded  together  with  a  spade,  but  the  process  is  slow 


FIG.  98.— Vertical  Pug-mill. 

and  imperfect.  A  better  result  is  obtained  by  treading  it 
with  bare  feet  or  by  turning  a  number  of  horses  on  to  it  and 
keeping  them  moving  about  until  the  mass  is  sufficiently 
uniform  ;  this  method  is  also  too  imperfect  in  highly  civilised 
lands. 

It  is  usual,  in  this  country,  to  employ  a  pug-mill  or  mixing 
machine  driven  by  horse-power  or  mechanically  according 
to  the  output  required.  Such  a  pug-mill  consists  of  a  closed 
cylinder  with  an  inlet  at  one  end  and  an  outlet  at  the  other, 


MIXING   THE   CLAY 


331 


and  provided  with  a  shaft  which  runs  right  through  the  centre 
and  is  fitted  with  blades  or  mixing  knives.  These  knives  are 
specially  designed  to  cut  the  clay  contained  in  the  cylinder  and 
to  mix  it  as  it  travels 
from  one  end  of  the 
machine  to  the  other. 
Pug-mills  were  for- 
merly of  the  vertical 
type,  the  clay,  etc., 
being  fed  in  at  the  top 
of  the  cylinder  and 
passing  out  near  the 
bottom,  but  at  the 
present  time  a  very 
large  number  of  hori- 
zontal pug-mills  are 
in  use  and  have  several 
advantages  when  em- 
ployed in  connection 
with  other  machinery. 
Moreover,  by  making 
the  pug-mill  horizon- 
tal a  portion  of  the 
cylindrical  casing  may 
be  omitted  and  an 
open  or  trough-mixer 
produced,  with  the 
advantage  that  the 
whole  of  the  mixing 
operations  may  be 
observed  and  any 
defective  blades  re- 
placed. It  is  also 
much  easier  to  regu- 
late  the  addition  of  Fl<3'  99.-Open  or  Trough-mixer. 

water  and,  therefore,  the  consistency  of  the  paste,  when  an 
open  mixer  is  employed. 

The  speed  of  rotation  of  the  shaft,  the  shape  and  number 
of  the  blades,  and  the  length  of  the  pug-mill  will  determine 


332  METHODS   OF   BRICKMAKING 

the  extent  of  the  kneading  and  the  resultant  texture  of  the 
pasty  mass.  Clays  and  mixtures  which  are  difficult  to  work 
may  require  to  be  passed  several  times  through  the  pug-mill, 
or  through  two  or  even  three  pug-mills  in  succession.  With 
most  brickmaking  clays  which  have  been  adequately  crushed 
or  ground,  a  well  designed  pug-mill  six  feet  in  length  will  be 
found  ample,  but  the  variation  in  character  of  clays  in 
different  localities  is  so  great  that  no  definite  limit  of  length 
can  be  laid  down. 

If  an  exceptionally  thorough  kneading  of  a  somewhat  lean 
clay  is  necessary  in  order  to  produce  a  clay  of  the  desired 
plasticity — as  is  the  case  with  some  mixtures  of  grog  and 
fireclay — an  .ordinary  pug-mill  is  not  always  efficient,  and  it 
is  then  better  to  employ  a  pan  mill  or  tempering  mill.  This 
machine  consists  of  an  edge-runner  mill  with  a  revolving  pan 
similar  to  that  used  for  grinding,  but  of  lighter  construction 
and  with  no  perforations  in  the  pan.  Machines  of  this  type 
are  well  known  under  the  term  mortar  mills,  as  they  are  largely 
used  for  mixing  mortar.  The  clay  together  with  such  other 
materials  as  are  to  be  mixed  with  it,  and  the  necessary  quantity 
of  water,  are  placed  in  the  pan  and  the  mill  is  set  in  action  for 
twenty  minutes  or  more,  according  to  the  amount  of  tempering 
required.  At  the  end  of  a  suitable  time  a  peculiarly  shaped 
shovel  mounted  on  a  swivel  is  employed  to  empty  the  mill, 
after  which  a  fresh  charge  is  added.  The  machine  works 
intermittently,  and  the  quality  of  the  product  will  depend 
chiefly  on  the  manner  in  which  the  materials  constituting  the 
charge  are  added.  With  a  little  care  the  paste  obtained  is 
remarkably  uniform,  and  is  somewhat  more  plastic  than  when 
the  same  materials  have  been  treated  in  a  pug-mill.  The  use 
of  a  tempering  mill  is,  however,  considerably  more  expensive, 
so  that  pug-mills  are  preferred  wherever  practicable. 

Consistency. — The  production  of  a  paste  of  the  required 
consistency  completes  the  second  stage  of  brickmaking.  The 
consistency  of  this  paste  varies  greatly  with  different  clays — 
some  needing  to  be  made  into  a  sloppy  material  scarcely  stiffer 
than  freshly-made  mortar,  whilst  others  are  so  stiff  that 
considerable  pressure  has  to  be  exercised  in  order  to  make  any 
impression  upon  them, 


MOULDING   BY    HAND  333 

The  softer  the  paste  the  easier  it  is  to  produce,  so  that  in 
districts  where  little  or  no  machinery  is  available,  and  the 
bricks  are  simply  moulded  by  hand,  the  paste  will  be  extremely 
soft.  Where  powerful  mechanical  mixers  are  available, 
however,  it  is  more  economical  to  employ  a  stiff er  paste. 

Methods  of  Shaping  the  Clay. — As  already  stated,  there  are 
a  number  of  different  methods  in  use  for  converting  the 
prepared  clay  or  mixed  material  into  bricks,  and  these  may 
now  be  considered  in  order. 

In  consequence  of  these  differences  in  consistency,  various 
methods  of  converting  the  paste  into  bricks  and  other  articles 
are  frequently  distinguished  by  the  kind  of  clay  paste  used. 
Thus— 

(a)  The   plastic  methods  of   brickmaking  employ   a  plastic 
paste,  as  in  the  manufacture  of  hand-made  and  wire-cut  bricks. 

(b)  The  semi-plastic  methods  of  brickmaking  employ  a  stiff 
paste,  and  are  sometimes  termed  "  stiff  plastic  "  processes. 

(c)  The  semi-dry  process  of  brickmaking  consists  in  the  use 
of  a  moistened  powder  which  is  almost  devoid  of  plasticity. 

(d)  In  the  dry  or  dust  process  an  almost  dry  powder  is  used. 
For  hand-made  bricks  the  paste  is  made  into  the  shape  of 

bricks  by  means  of  wooden  or  metal  moulds,  consisting  of  a 
stout  box,  without  lid  or  bottom,  which  rests  on  a  piece  of 
hardwood  fastened  to  a  rough  table.  The  base  piece  is  some- 
times covered  with  a  special  kind  of  cloth,  to  which  the  paste 
adheres  so  slightly  that  the  mould  with  its  contents  can  be 
readily  lifted  off.  Without  the  use  of  such  a  cloth  or  of  sand 
sprinkled  on  the  table  the  clay  paste  would  adhere  tenaciously 
to  the  table.  If  the  bricks  are  to  have  a  frog  or  cavity  on  one 
side,  a  piece  of  wood  or  brass  is  fixed  to  the  table  or  base  piece, 
and  guides  or  pegs  must  then  be  used  to  secure  the  mould 
always  being  correctly  placed  on  the  bench.  It  is  less  easy 
to  produce  a  frog  on  a  slop-moulded  than  on  a  sand-moulded 
brick. 

The  moulder  prepares  the  mould,  places  it  in  a  convenient 
position  on  the  table,  and  then  takes  up  a  convenient  quantity 
of  the  soft  paste  in  both  hands,  raises  it  above  his  head  and 
throws  it  down  with  great  force  into  an  empty  mould  placed 
on  a  bench  or  table  in  front  of  him.  He  then  presses  the  paste 


334  METHODS   OF   BRICKMAKING 

well  into  the  corners  of  the  mould,  scrapes  off  any  superfluous 
paste  with  a  wooden  blade  or  "  strike,"  and  with  a  dextrous 
turn  of  the  wrist  he  empties  the  contents  of  the  mould  on  to  a 
small  board  or  pallet  placed  convenient  for  its  reception  by 
the  moulder's  assistant.  The  brick  thus  produced  is  carried 
away  to  be  dried,  either  by  hand  or  on  a  barrow  of  special 
construction,  so  as  to  avoid  undue  vibration  of  the  bricks. 
Meanwhile,  the  moulder  dips  his  mould  in  water  so  as  to  wet 
it  thoroughly  (slop  moulding),  or  first  in  water  and  then  in 
sand  (sand  moulding).  If  sand  is  used  it  will  cling  to  the 
surfaces  of  the  clay  in  contact  with  the  mould  and  will  produce 
a  rough-faced  brick,  the  colour  of  which  will  depend  upon  that 
of  the  sand  when  burned.  As  red-burning  sand  is  generally 
employed  for  this  purpose,  sand-faced  or  sand-moulded  bricks 
are  usually  of  a  good  red  colour  when  sold,  whilst  slop-moulded 
bricks  are  the  same  colour  as  the  burned  clay  of  which  they 
are  made. 

Although  the  difference  between  sand-  and  slop-moulding 
appears  to  be  slight,  in  practice  they  necessitate  an  entirely 
different  arrangement  of  the  works.  Slop-moulded  bricks  are 
so  soft  that  they  must  usually  be  carried  one  at  a  time  and 
placed  about  an  inch  apart  on  a  level  floor  until  they  have 
hardened  slightly  and  can  then  be  taken  to  the  hacks.  Sand- 
moulded  bricks,  on  the  contrary,  are  stiffer  and  stronger,  and 
can  be  taken  in  quantities  on  barrows  and  stacked  directly 
on  the  hacks — one  sand-moulder  keeping  two  men  and  three 
barrows  constantly  employed  in  the  transport  of  the  bricks. 

When  slop-moulded  bricks  are  made,  the  drying  floor  must 
be  close  to  the  moulding  bench — it  is,  indeed,  customary  for 
the  bench  to  be  moved  to  different  parts  of  the  floor  two  or 
three  times  each  day — but  sand-moulded  bricks  can  be  taken 
on  a  barrow  for  any  reasonable  distance.  In  works  provided 
with  a  steam-heated  drying  floor,  slop -moulding  is  generally 
used,  particularly  if,  as  in  the  case  of  bricks  for  inside  work, 
the  colour  of  the  finished  bricks  is  of  little  importance. 

The  rate  at  which  hand-made  bricks  can  be  moulded  is  very 
great,  a  fair  average  being  36,000  or  more  sand-moulded,  or 
9,000  to  10,000  slop-moulded,  bricks  per  week. 

Each  brick  when  freshly  moulded  contains  about   1  Ib.  of 


MOULDING  BY   HAND  335 

water,  and  this  must  be  removed  by  drying  in  such  a  manner 
that  the  brick  is  not  damaged.  Slop-moulded  bricks  are 
usually  allowed  to  remain  on  the  flat  or  floor  for  about  six 
days,  after  which  they  may  be  stacked  in  an  open  fashion 
in  long  rows  about  six  bricks  high,  so  as  to  make  more  room 
for  fresh  bricks.  They  remain  stacked  for  several  weeks 
until  dry  enough  to  be  burned.  Sand-moulded  bricks  are 
stiffer  and  are  arranged  in  hacks  (Fig.  110)  immediately  after 
they  have  left  the  mould,  and  take  much  longer  to  dry.  These 
hacks  consist  of  long  rows  of  bricks  set  openly  one  above  the 
other  to  a  height  of  about  two  feet,  and  are  covered  with  gable- 
shaped  boards  to  keep  off  the  rain.  The  sides  of  the  hacks  are 
also  protected,  when  necessary,  with  loo-boards,  matting  or 
straw  in  order  that  the  bricks  may  not  be  damaged  by  frost, 
draughts  or  water.  The  bricks  must  usually  be  taken  down 
and  rearranged  at  least  once  during  the  drying,  arid  if  pressed 
bricks  are  desired  a  portable  press  is  taken  to  the  hacks  and  the 
bricks  pressed  and  replaced. 

The  drying  of  bricks  always  requires  care  and  attention  ; 
apparently  insignificant  draughts  will  crack  many  bricks, 
and  even  if  the  sun  shines  on  some  bricks  during  drying 
the  damage  will  be  serious.  Hence,  a  considerable  proportion 
of  the  anxiety  experienced  by  the  owner  of  a  yard  where 
hand-made  bricks  are  produced  is  due  to  the  difficulty  of 
avoiding  loss  during  the  drying,  especially  if  the  clay  is  a  tender 
one. 

The  thoroughly  dry  bricks  are  next  taken  to  a  clamp  or  kiln 
to  be  burned  as  described  later. 

The  difficulties  experienced  in  obtaining  skilled  brick-moulders 
has  led  to  the  introduction  of  machines  in  which  the  hand- 
moulding  process  is  closely  imitated.  These  machines  are 
described  in  the  author's  "  Modern  Brickmaking "  (Scott 
Greenwood  &  Son),  but  they  have  not  been  used  extensively, 
as  others  working  on  entirely  different  principles  have  a  much 
larger  output. 

Wire-cut  bricks  are  produced  by  machinery ;  they  are  not 
moulded,  but  are  shaped  by  expression  through  a  suitable  die 
in  a  manner  greatly  resembling  the  production  of  sausages. 
The  plastic  paste  (prepared  in  one  of  the  ways  previously 


336  METHODS    OF   BRICKMAKING 

described)  is  passed  from  the  mixing  machine  into  a  pug-mill . 
the  die  being  attached  to  the  exit  end  of  the  latter,  and  thus 
forming  a  mouthpiece,  through  which  the  clay  exudes  in  the 
shape  of  a  band  or  column  9|  inches  by  4|  inches,  i.e., 
whose  width  is  the  length  of  a  brick,  and  whose  thickness  is 
the  width  of  a  brick.  This  band  is  cut  into  convenient 
lengths  by  means  of  a  wire  stretched  tightly  in  a  frame,  and 
each  section  is  again  cut  into  pieces  about  2-J  inches  or 
3  inches  wide  by  a  series  of  other  stretched  wires.  In  this 
manner  the  bricks  are  produced  six  or  more  at  a  time,  and 
are  taken  away  on  long  boards  or  pallets  to  be  dried. 

This  process  of  brickmaking  is  exceedingly  simple  in  theory, 
but  there  are  numerous  matters  in  connection  with  it  which 
require  skill  and  care  if  good  bricks  are  to  be  produced.  Thus, 
the  construction  and  maintenance  of  the  mouthpiece  need 
constant  attention,  or  the  clay  band  will  be  irregular  in  shape 
and  having  serrated  edges.  Some  clays  are  extremely  trouble- 
some in  this  respect  and  have  to  be  passed  between  a  pair  of 
expression  rolls,  placed  between  the  pug-mill  and  the  die, 
before  good  bricks  can  be  obtained. 

The  arrangement  of  the  wires  on  the  cutting  table  also 
admits  of  numerous  modifications,  some  of  which  are  far  better 
than  others.  The  usual  plan  in  this  country  is  to  keep  the  wires 
fixed  and  to  push  the  clay  column  sideways  through  them  by 
means  of  a  push  plate,  but  much  neater  bricks  can  be  obtained 
lay  moving  the  frame  carrying  the  wires  in  a  diagonal  direction 
towards  the  table,  as  in  most  of  the  Continental  machines. 

Any  stones  present  in  the  clay  band  may  catch  the  wires 
and  tear  the  bricks,  so  that  the  wire-cut  process  is  not  well 
adapted  for  very  rough  clays,  though  excellent  for  most  others. 

Owing  to  the  manner  of  their  production,  wire-cut  bricks 
cannot  be  provided  with  frogs  or  depressions  unless  the  bricks 
are  passed  through  a  re -press. 

In  order  to  reduce  the  space  occupied  by  the  plant,  some 
of  the  makers  of  brick  machinery  combine  the  crushing  rolls, 
mixer,  pug-mill,  mouthpiece,  and  expression  rolls  on  a  single 
framework  so  that  the  whole  plant  has  the  appearance  of  a 
single  machine. 

Bricks  made  by  the  wire-cut  process  are  very  soft,  and  must 


THE   WIRE-CUT   PROCESS 


337 


usually  be  laid  out  on  a  drying  floor  or  kept  separate  on  the 
racks  of  a  drying  tunnel  or  "  stove  "  in  order  that  they  may 


become  dry  and  hard.     The  most  generally  employed  arrange- 
ment for  drying  consists  of  a  large  concreted  floor  in  a  corre- 
ct z 


338  METHODS    OF   BRICKMAKING 

spondingly  large  and  well-ventilated  shed.  Beneath  the  floor 
is  a  series  of  flues  heated  by.  steam  or  fires  in  such  a  manner 
that  the  temperature  of  the  floor  is  as  uniform  as  possible. 
Steam  has  several  advantages  over  fires,  particularly  in  the 
regulation  of  the  temperature  of  various  parts  of  the  floor. 
Whichever  source  of  heat  is  employed,  the  bricks  are  placed 
singly  on  the  floor  about  f  inch  apart  and  care  is  taken  to  avoid 
draughts  and  to  raise  the  temperature  very  steadily.  In  three 
to  five  days  the  bricks  are  usually  sufficiently  dry  to  be  taken 
to  the  kiln. 

Where  the  output  of  bricks  is  sufficiently  large  a  tunnel 
dryer  may  be  employed  and,  if  rightly  constructed,  will  be 
more  efficient  than  a  drying  floor.  Unfortunately,  however, 
many  of  the  tunnel  dryers  now  in  use  are  far  from  satisfactory 
owing  to  the  lack  of  knowledge,  on  the  part  of  both  designers 
and  users,  of  the  principles  underlying  the  construction.  In  a 
tunnel  dryer  the  bricks  are  placed  on  cars  and  enter  one  end 
of  a  tunnel,  travel  along  it  to  the  other  end  and  finally  emerge, 
after  twenty-four  to  seventy  hours,  in  a  dry  state.  The  sim- 
plicity of  the  operation,  the  reduction  in  the  amount  of  handling, 
and  the  lower  cost  of  heating  are  all  in  favour  of  the  use  of 
tunnel  dryers,  but  at  this  and  in  all  stages  of  brickmaking, 
the  manufacture  is  not  as  simple  as  it  appears  to  be,  and  both 
care  and  skill  are  needed  in  the  management  of  the  temperature 
and  ventilation  of  the  tunnels. 

The  use  of  some  means  of  drying  plastic  clays  before  they 
enter  the  kiln  is  imperative,  as  otherwise  the  bricks  would 
crack  and  fall  to  pieces  in  the  kiln.  The  details  of  design  in 
a  drying  plant  suited  to  a  particular  clay  must  be  adapted  to 
the  special  needs  of  that  clay  ;  it  is  no  more  reasonable  to 
expect  to  dry  a  clay  efficiently  in  a  dryer  which  has  not  been 
made  to  suit  it  than  it  is  for  a  man  to  expect  to  be  well  dressed 
in  a  ready-made  suit  of  clothes,  or  for  him  to  be  suited  with  the 
first  hat  he  tries  on. 

If,  in  spite  of  all  care  in  drying  and  in  the  use  of  a  dryer 
of  suitable  design,  the  proportion  of  bricks  which  crack  con- 
tinues to  be  large,  there  is  a  probability  that  the  clay  is  too 
plastic  and  that  it  requires  to  be  diluted  with  sand  or  some 
other  non-plastic  material.  The  impossibility  of  obtaining 


THE   WIRE-CUT    PROCESS  339 

sand  at  a  sufficiently  cheap  rate  is  one  of  the  chief  reasons 
why  numerous  clays — otherwise  suitable — cannot  be  used 
for  brickmaking. 

If  the  cracks  appear  to  emanate  from  the  edges  of  any  brand 
or  other  distinguishing  mark  stamped  on  the  bricks  or  formed 
on  them  during  the  moulding,  the  texture  of  the  material 
requires  adjustment.  If  a  coarse  material  is  used,  cracks  are 
almost  certain  to  be  formed  wherever  there  is  an  indentation 
in  the  bricks.  The  coarser  particles  act  as  centres  of  radiation 
for  the  cracks. 

After  being  dried,  the  bricks  are  taken  to  a  kiln  and  burned 
in  a  manner  to  be  described  later.  There  is  often  much 
unrecognised  carelessness  in  drying  which  results  in  the  pro- 
duction of  numerous  hair-like  cracks  in  the  bricks.  These 
cracks  are  almost  invisible  in  the  unfired  bricks,  and  their 
occurrence  in  the  finished  bricks  is,  for  this  reason,  often 
wrongly  attributed  to  the  action  of  the  kiln.  Further  details 
of  the  manufacture  of  bricks  from  a  plastic  paste  will  be  found 
in  the  author's  "  Modern  Brickmaking  "  (Scott,  Greenwood  & 
Son). 

In  the  semi-plastic  or  stiff-plastic  methods  of  brickmaking  a 
paste  of  such  stiffness  is  employed  that  very  considerable 
pressure  has  to  be  used  in  order  to  obtain  the  imprint  of  a 
thumb  or  finger.  This  very  stiff  paste  is  usually  prepared 
from  a  powdered  shale  or  other  indurated  clay,  as  the  variations 
in  stiffness  of  clays  quarried  in  a  stiff  plastic  condition  make 
their  use  inconvenient. 

The  crushed,  screened  and  powdered  clay  (p.  329)  is  received 
in  an  open  mixer,  and  is  there  kneaded  with  the  requisite 
quantity  of  water  and  passed  into  a  small  but  powerful  pug- 
mill  which  compresses  it  into  metal  moulds.  These  moulds 
may  be  arranged  on  the  top  of  a  rotating  table  or  on  the 
circumference  of  a  drum,  both  these  constructions  having 
proved  satisfactory.  The  rough-shaped  bricks  or  clots  are 
then  removed  from  the  moulds,  one  at  a  time,  and  are  pressed 
accurately  to  shape  in  a  plunger-press,  which  is  attached  to 
the  same  framework.  The  shape  of  the  moulds  and  the 
arrangements  provided  for  removing  the  clots  from  them  has 
a  great  influence  on  the  power  required  by  the  machine,  it 

z  2 


340 


METHODS   OF   BRICKMAKING 


being  generally  found  that  the  simpler  the  clot  mould  the 
better.  Thus,  in  the  Bradley  and  Craven  stiff -plastic  brick 
machine  the  moulds  are  rectangular  depressions  in  a  horizontal 
steel  disc  ;  in  the  Scholefield  machine  they  form  similar 
depressions  in  the  circumference  of  a  drum,  and  in  the  Fawcett 
machine  they  form  the  "  spaces  "  between  the  "  cogs  "  in 
a  peculiarly  shaped  "  cogged  wheel."  In  the  two  first- 


FIG.  101. — Bradley^and  Craven  Stiff-plastic  Brick-machine. 

mentioned  machines  the  clay  is  pressed  upwards  or  outwards, 
but  in  the  last-named  one  it  is  pressed  longitudinally. 

Each  machine  is  rated  at  about  12,000  bricks  per  day,  but 
the  actual  output  depends  on  the  nature  of  the  clay. 

The  advantages  of  using  so  stiff  a  paste  are  twofold  ;  the 
clay  is  obtained  in  a  condition  suitable  for  immediate  pressing 
into  shape,  and  the  bricks  may  usually  be  sent  direct  to  the 


THE   WIRE-CUT   PROCESS  341 

kiln  without  the  need  of  drying.  A  considerable  saving  is 
thereby  effected,  as  it  is  difficult  to  watch  the  drying  of  plastic 
bricks  so  closely  as  to  be  able  to  re-press  them  when  all  are  in 
the  best  condition  for  this  operation,  and,  further,  the  cost  of 
a  dryer  is  completely  avoided,  though  against  this  there  is  the 
cost  of  additional  fuel  required  in  the  kiln.  Where  a  continuous 
kiln  is  not  available  these  bricks  must,  in  some  instances,  be 
dried  before  being  placed  in  intermittent  kilns.  The  burning 
is  carried  out  as  described  later. 

Further  details  of  the  machines  used  for  making  stiff-plastic 


FIG.  102. — Fawcett  Stiff -plastic  Brick  Machine. 

bricks  will  be  found  in  the  author's  "  Modern  Brickmaking  " 
(Scott,  Greenwood  &  Son). 

In  the  semi-dry  process  the  material  is  used  in  the  form  of 
a  powder  which  contains  just  sufficient  moisture  to  make  it 
"  cake."  It  can  only  be  used  with  shales  and  dry  clay  which 
are  almost  devoid  of  plasticity.  The  powder  is  obtained  by 
crushing  the  material  with  edge  runners,  as  already  described 
(p.  329),  and  is  fed  into  the  boxes  of  plunger  presses  of  a 
particularly  powerful  type,  in  which  it  is  compressed  into 
bricks.  Several  types  of  press  are  in  use  ;  in  each  case  they 
must  be  capable  of  exerting  an  enormous  pressure  and  of 
giving  several  compressions  in  succession,  as  a  single  pressure, 


342 


METHODS   OF   BRICKMAKING 


however  great,  will  not  produce  a  sound  brick.  It  appears 
to  be  necessary  to  press  once,  release  the  pressure  and  allow 
air  to  escape,  re-press,  remove  from  the  mould,  and  again 
re-press  either  once  or  twice  before  a  reliable  brick  can  be  made 
from  some  materials. 

The  semi-dry  process  has  gained  its  chief  reputation  in  the 
neighbourhood  of  Accrington  (where  it  is  now  being  replaced 


FIG.  103. — Whittaker  Plunger  Press. 

by  stiff-plastic  machines)  and  Peterborough,  where  enormous 
quantities  are  made  annually.  It  is  claimed  that,  where  it  is 
applicable,  this  is  the  cheapest  of  all  brickmaking  processes 
when  large  quantities  are  required,  though  the  bricks  produced 
are  less  readily  purchased  by  builders  on  account  of  their  low 
porosity  and  their  consequent  tendency  to  ':  float  "  on  mortar. 
This  process  ought  never  to  be  installed  except  under  expert 
advice  which  is  quite  independent  of  that  of  the  various 


RE-PRESSING   BRICKS.  343 

makers  of  machinery,  or  serious  disappointment  may  result. 
It  is  therefore  unnecessary  to  describe  it  in  greater  detail  here, 
but  a  fuller  description  of  it  will  be  found  in  the  author's 
"  Modern  Brickmaking "  (Scott,  Greenwood  &  Son).  As 
suggested,  bricks  made  by  the  semi-dry  process  contain  so 
little  moisture  that  they  are  sent  direct  to  the  kiln  and  do  not 
need  to  be  dried. 

The  dry  dust  process  is  seldom  used  for  bricks,  as  the  diffi- 
culties experienced  in  obtaining  sound  bricks  are  very  great. 
For  tiles  and  other  thin  articles  it  is  largely  used.  As  the 
name  implies,  the  clay  or  shale  is  ground  to  the  form  of  a  dust 
and  this  is  placed  in  the  box  of  the  press  and  is  duly  compressed 
into  the  desired  shape  in  a  manner  similar  to  bricks  made  by 
the  semi-dry  process.  The  difficulty  of  removing  all  air  from 
between  the  particles  and  of  exercising  a  perfectly  uniform 
pressure  over  every  part  of  the  brick  is  so  great  as  to  make  the 
use  of  a  semi-dry  material  necessary  for  brickmaking.  Indeed, 
the  machines  which  are  supposed  to  be  making  bricks  from  dry 
dust  are,  in  almost  every  case,  working  with  a  slightly  moistened 
material  by  means  of  the  semi-dry  process. 

Re-pressing  Bricks. — Facing  bricks,  or  those  in  which  great 
exactitude  of  shape  and  size  is  essential,  are  not  infrequently 
placed  in  a  re-press,  and  any  irregularities  in  form  are  thereby 
corrected.  Re-pressing  is  costly,  as  even  with  a  power-driven 
press  two  strong  youths  cannot  deal  with  more  than  2,000 
bricks  a  day,  and  even  at  this  rate  a  considerable  number  of 
bricks  will  be  defective.  Unless  the  clay  is  in  exactly  the  right 
condition  the  number  of  bricks  which  can  be  satisfactorily 
re-pressed  will  be  very  small.  Too  moist  a  paste  will  adhere 
too  closely  to  the  press  box  and  plunger,  no  matter  how  well 
they  are  oiled,  and  too  dry  a  clay  will  break  and  crack,  thereby 
producing  bricks  of  an  unsightly  appearance  when  burned. 
For  this  reason,  where  re-pressing  is  practised,  both  youths  and 
foremen  must  be  very  alert  as  to  the  condition  of  the  clay,  and 
must  be  regardless  of  overtime  if  a  large  number  of  bricks  is  in 
the  desired  condition. 

Much  difference  of  opinion  exists  as  to  the  real  value  of 
re-pressing.  If  the  alteration  in  the  shape  of  the  brick  or  clot 
is  appreciable,  the  structure  of  the  brick  will  be  damaged 


344  METHODS   OF    BRICKMAKING 

and  the  crushing  strength  seriously  reduced.  Where  appear- 
ance and  accuracy  of  shape  are  of  such  importance  that  a 
slight  loss  of  strength  need  not  be  considered,  re-pressing 
may  be  desirable.  It  is,  however,  quite  a  mistake  to  suppose 
that  re -pressing  really  improves  bricks  ;  it  may  give  them  a 
better  appearance,  but  it  can  only  reduce  their  strength. 
Hence,  the  able  brick  manufacturer  endeavours  to  make  his 
bricks  right  at  first  and  so  to  avoid  the  necessity  of  re-pressing. 

Burning  Bricks. — In  some  countries,  where  the  climatic 
conditions  are  favourable,  bricks  are  simply  placed  in  the  sun, 
the  rays  from  which  are  sufficiently  intense  to  effect  a  sufficient 
hardening  of  the  mass.  Sun-baked  bricks  have  so  low  a  resist- 
ance that  they  are  far  from  durable,  so  that  one  of  the  early 
results  of  civilisation  is  the  substitution  of  bricks  baked  by 
means  of  fuel.  In  cooler  climates  the  use  of  fuel  is  a  necessity 
in  the  production  of  sound  bricks. 

Pieces  of  clay  in  the  shape  of  bricks,  but  which  have  not 
been  heated  are  termed  green  bricks,  this  term  being  analogous 
to  that  applied  to  freshly  cut  wood.  They  cannot  be  used 
for  building  in  moist  climates,  as  they  would  be  washed  to  pieces 
by  a  heavy  shower  of  rain  beating  upon  them.  To  render 
them  permanent  they  must  be  heated  to  a  temperature 
sufficiently  high  to  make  them  durable. 

When  a  piece  of  plastic  clay  is  first  allowed  to  dry  thoroughly 
and  is  then  heated  slowly  and  steadily  to  a  bright  red  heat, 
a  number  of  remarkably  interesting  changes  take  place  in 
both  its  chemical  and  physical  properties.  These  are  described 
more  fully  in  a  separate  chapter,  but  the  appliances  used  for 
effecting  these  changes  may  be  briefly  described  here. 

Two  chief  groups  of  appliances  are  used  for  heating  bricks  : 

(a)  Clamps,  which  consist  of  a  peculiar  stacking  of  the  bricks 
and  fuel,  covering  the  outside  of  the  stack,  or  clamp,  with  clay 
paste  and  then  lighting  the  fuel.  Under  favourable  conditions 
the  heat  produced  will  burn  the  raw  clay  into  good  and  service- 
able bricks. 

Clamps  are  particularly  suitable  for  small  outputs,  as  there 
is  no  outlay  for  the  construction  of  a  kiln,  but  the  bricks 
produced — whilst  strong  and  durable — have  not  the  pleasant 
appearance  of  kiln-burned  bricks. 


BURNING   BRICKS  345 

The  construction  of  a  clamp  requires  too  much  skill  for  a 
detailed  description  to  be  given  here.  It  consists  essentially 
in  building  a  series  of  flues,  and  above  these  the  bricks  are 
stacked  close  together,  the  direction  of  the  courses  changing 
frequently.  Coke  breeze  is  laid  in  the  flues  and  fine  coke 
or  cinders  is  sprinkled  over  each  course  of  bricks  ;  moreover, 
the  bricks  to  be  burned  in  clamps  usually  have  a  considerable 
percentage  of  coke  or  cinder  dust  mixed  with  the  clay  before 
it  is  used  ;  this  forms  a  further  supply  of  fuel,  with  the  result 
that  when  a  clamp  is  burning  briskly  each  brick  becomes  a 
sort  of  fireball,  and  the  heating  is  so  effective  that  only  a  very 
small  proportion  of  fuel  is  needed.  "  London  stocks  "  are 
typical  clamp-burned  bricks. 

(b)  Kilns  or  ovens,  which  consist  of  brickwork  chambers 
into  which  the  bricks  are  placed,  the  fuel  being  usually  kept 
separate  and  burned  in  specially  designed  fireplaces  or  furnaces. 
Bricks  burned  in  kilns  have  a  more  uniform  and  pleasing 
colour  than  those  heated  in  clamps,  and  are  of  more  uniform 
strength,  but  the  cost  of  the  kiln  itself  and  of  the  additional 
fuel  required  enables  clamp-burned  bricks  still  to  hold  their 
own  in  districts  where  the  clay  or  brick-earth  is  favourable. 
The  number  of  different  patterns  of  kilns  is  incalculable, 
but  they  can  all  be  grouped  under  four  heads,  the  kilns  in  each 
group  differing  from  each  other  in  details  which,  however, 
important  in  practice,  cannot  be  adequately  described  here. 

(1)  Single  up-draught  kilns  of  rectangular  shape,  commonly 
termed  Scotch  kilns,  are  formed  by  building  four  walls  around 
a  suitably  sized  space,  and  providing  a  narrow  doorway  at 
each  end  and  fireplaces  along  each  of  the  longer  sides.  The 
top  of  the  kiln  is  quite  open  and  usually  there  is  no  chimney  ; 
though  a  series  of  small  stacks — one  to  each  fire — may  be 
constructed  if  necessary.  These  kilns  are  filled  with  bricks 
carried  through  the  doors  or  (in  America)  by  means  of  a  crane 
through  the  open  top  of  the  kiln.  The  bricks  are  stacked  in 
a  special  manner  about  one  finger's  width  apart,  the  courses 
being  crossed  so  as  to  obtain  the  most  uniform  distribution 
of  the  heat.  The  top  of  the  kiln  is  covered  with  old  bricks 
and  earth  or  ashes.  Kilns  of  this  type  have  long  been  in  regular 
use  in  almost  every  county  north  of  the  Trent,  but  they  are 


346 


METHODS    OF   BRICKMAKTNG 


fuel  wasters  and  are  being  replaced  by  continuous  kilns  wherever 
the  output  justifies  the  erection  of  the  latter. 

(2)  Single  down-draught  kilns  are  either  circular  or  rectangular 
in  plan,  and  are  covered  with  an  arched  roof  or  dome.     The 


FIG.  104.— Single  Up -draught  Kiln. 

fires  are  placed  at  regular  intervals  in  the  walls  and  the  fire 
gases  first  rise  up  to  the  roof  of  the  kiln  and  are  then  deflected 
downwards  and  distributed  among  the  goods  until  they  eventu- 
ally pass  out  through  flues  in  the  floor  of  the  kiln  to  the  chimney 


FIG.  105. — Single  Down-draught  Kiln. 

stack.  The  great  advantage  of  a  well-built  down-draught 
kiln  is  that  facing  bricks  of  best  quality  can  be  burned  in  it, 
the  colour  of  the  bricks  not  being  damaged  by  the  flame  as 
in  kilns  with  up-draught  or  horizontal  draught.  Consequently, 


BURNING  BRICKS 


347 


for  small  yards  where  facing  bricks,  tiles  and  terra-cotta  are 
made,  single  down-draught  kilns  are  essential  for  these  goods. 
In  larger  works,  continuous  kilns  of  a  corresponding  type  are 
preferable. 

(3)  Single  horizontal  draught  kilns  are  usually  known  as 
"  Newcastle  kilns  "  and  are  rectangular  in  plan.  In  outward 
appearance  they  resemble  a  rectangular  down-draught  kiln 
or  a  Scotch  kiln,  to  which  a  roof  has  been  attached,  but  the 
fireplaces  are  arranged  differently.  In  a  Newcastle  kiln  the 
fires  are  all  at  one  end,  and  the  flames  and  hot  gases  travel 
horizontally  through  the  goods  to  the  opposite  end,  where  they 
pass  into  a  flue  leading  to  the  chimney.  It  is  customary  to 


'   Fire-places  at  end  ' 

FIG.  106.— Newcastle  Kiln. 

arrange  Newcastle  kilns  in  series  back  to  back,  and  when  very 
large  ones  are  used  fires  are  built  at  both  ends  whilst  the  waste 
gases  pass  away  at  the  centre.  A  separate  chimney  may  be 
built  for  each  kiln,  but  where  several  kilns  are  in  use  it  is  better 
to  employ  a  single  chimney  stack  for  all  the  kilns.  Newcastle 
kilns  are  largely  used  for  burning  firebricks,  though  circular 
down-draught  kilns  are  sometimes  preferred  for  this  purpose. 
(4)  Continuous  kilns  are  really  down-draught  and  horizontal- 
draught  kilns,  in  which  the  waste  gases  from  one  kiln  are  not 
sent  direct  to  the  chimney,  but  are  passed  through  a  number  of 
other  kilns  or  "  chambers  "  until  the  temperature  of  the  gases 
is  too  low  to  be  of  further  use.  In  these  kilns  the  gases  passing 


348  METHODS    OF   BRICKMAKING 

up  the  chimney  should  never  have  a  temperature  exceeding 
130°  C.,  whereas  those  from  single  kilns  frequently  have  a 
temperature  of  800°  C.  or  even  higher,  thus  wasting  about 
half  the  fuel  used.  In  a  continuous  kiln,  on  the  contrary,  this 
loss  of  fuel  is  prevented,  as  the  heat  which  is  not  required  to 
burn  the  goods  to  which  it  is  first  applied  is  utilised  to  warm 
up  goods  in  other  parts  of  the  structure. 

This  method  of  heating  is  primarily  due  to  Siemens,  who 
adopted  it  in  connection  with  the  steel-melting  furnace  which 
bears  his  name.  He  observed  that  a  large  amount  of  heat  was 
passing  away  from  the  furnace  unused,  and  sought  to  utilise 
this  by  passing  the  waste  gases  through  masses  of  checkered 
brickwork.  When  one  such  mass  of  brickwork  was  sufficiently 
heated  the  waste  gases  were  diverted  to  another  "  regenera- 
tor "  and  air  was  drawn  in  the  opposite  direction  through  the 
hot  brickwork.  In  this  way  hot  air  was  supplied  for  the 
combustion  of  the  fuel,  and  a  great  saving  in  heat  was  effected. 

Hoffmann,  who  modified  Siemens'  furnace  and  applied  it  to 
brick-burning,  soon  saw  that,  instead  of  heating  up  permanent 
structures  with  the  waste  gases,  the  best  result  could  be 
secured  by  using  the  bricks  to  be  burned  as  regenerators,  and 
accordingly  devised  the  continuous  kiln  which  bears  his  name. 
Many  modifications  of  Hoffmann's  original  kiln  have  been 
made,  and  some  of  them  are  great  improvements  on  it  where 
the  colour  of  the  finished  bricks  is  of  importance,  but  in  none 
of  these  improvements  is  the  departure  from  the  Siemens- 
Hoffmann  regenerative  principle  very  great,  and  most  of  the 
newer  kilns  may  justly  be  regarded  as  adaptations  of  this 
principle  to  suit  the  special  circumstances  either  as  to  the 
product  or  the  fuel. 

Wherever  the  output  of  a  brickyard  exceeds  15,000  bricks 
per  day  or  450,000  per  year — and  in  some  cases  for  even  smaller 
outputs — a  properly  designed  continuous  kiln  will  be  found  to 
require  only  one-third  to  one-half  the  fuel  needed  by  the 
corresponding  number  of  single  kilns,  whilst  the  product  is  of 
equal  value  in  every  respect.  There  is  a  general  impression 
that  continuous  kilns  can  only  be  used  for  common  bricks  ; 
this  is  quite  erroneous,  as  the  best  terra-cotta  and  facing  bricks 
may  be  advantageously  burned  in  continuous  kilns.  The 


BURNING  BRICKS  349 

nature  of  the  product  depends  entirely  on  the  suitability  of 
the  kiln  for  the  purpose,  and  not  on  the  "  continuous  "  as 
distinct  from  "  single  "  kilns.  The  selection  of  a  kiln  is,  of 
course,  a  matter  requiring  expert  knowledge,  and  even  kiln 
builders  themselves  cannot  be  relied  upon  too  implicitly  in 
this  direction,  as  they  are  naturally  biassed  in  favour  of  the 
particular  designs  used  by  their  firms  and  cannot  be  expected 
to  admit  that  the  kilns  built  by  any  other  firm  are  more  suitable 
for  a  particular  case. 

The  following  pages  do  not  aim  at  presenting  more  than  a 
mere  outline  of  typical  continuous  kilns  for  various  classes  of 
bricks.  They  merely  show  the  general  principles  of  construc- 
tion, and  will  require  modification  to  local  circumstances  and 
requirements. 

The  original  Hoffmann  kilns  were  circular  in  plan,  but  it  is 
found  more  convenient  to  adopt  the  shape  shown  in  Fig.  3. 
The  Hoffmann  kiln,1  as  now  used,  consists  of  a  long  brickwork 
structure  containing  a  kind  of  endless  tunnel  from  which  a 
number  of  flues  lead  to  a  long  central  "  main  flue,"  the  latter 
being  connected  directly  to  the  chimney.  Each  of  these  flues 
is  controlled  by  a  separate  damper. 

In  the  "  roof  "  of  the  '"'  tunnel  "  is  a  series  of  rows  of  5-inch 
openings,  each  row  being  about  three  feet  apart  and  consisting 
of  three,  four  or  five  holes.  These  openings  are  covered  with 
air-tight  metal  caps,  and  are  known  as  feed  holes  ;  through 
them  the  fuel  is  introduced  into  those  portions  of  the  kiln, 
whilst  cold  air  is  admitted  through  those  parts  which  require 
cooling.  Larger  openings — termed  "  wickets  "  or  "  door 
gaps  " — are  made  in  the  outer  walls  of  the  kiln  and  serve  for 
the  admission  and  removal  of  the  bricks  to  be  burned.  The 
number  of  these  wickets  depends  on  the  size  of  the  kiln  : 
usually  there  is  one  to  each  fourteen  feet  of  linear  kiln  wall. 
A  Hoffmann  kiln  of  convenient  size  will  have  sixteen  wickets 
and  an  equal  number  of  dampers  leading  to  the  main  flue. 

As  a  matter  of  convenience,  it  is  desirable  to  regard  such  a 
kiln  as  composed  of  a  certain  number  of  units  or  chambers, 
each  of  which  corresponds  to  one  wicket  and  one  flue  damper. 

1  Hoffmann's  patents  have  long  since  expired,  and  most  firms  of  kiln  builders  are 
prepared  to  build  "  Hoffmann  kilns."' 


350  METHODS   OF  BRICKMAKING 

Thus,  a  kiln  with  sixteen  wickets  is  termed  a  sixteen-chamber 
kiln,  and  is  treated  as  though  it  were  actually  partitioned  off 
into  sixteen  separate  compartments,  though,  in  reality,  no 
such  partitions  exist  in  a  true  Hoffmann  kiln.1  A  smaller 
number  than  sixteen  wickets  is  seldom  desirable,  though  many 
kilns  have  only  twelve  or  fourteen.  They  are,  however,  less 
economical  in  fuel  than  sixteen-chamber  kilns,  as  the  larger 
number  of  chambers  permits  a  greater  utilisation  of  the  heat. 
It  is,  indeed,  a  very  false  idea  of  economy  to  erect  a  continuous 
kiln  with  a  small  number  of  chambers.  If  the  output  is  to 
be  small,  the  kiln  should  be  made  narrower  than  otherwise, 
i.e.,  the  distance  from  the  wickets  to  the  centre  of  the  kiln 
should  be  made  less,  but  the  effective  perimeter  of  the  kiln 
should  never  be  less  than  224  feet.  In  other  words,  the  output 
of  a  continuous  kiln  should  not  be  made  to  depend  on  the 
number  of  the  chambers,  but  on  their  width.  Failure  to 
recognise  this  simple  fact  is  the  cause  of  much  of  the  waste  of 
fuel,  and  most  of  the  disappointment  which  has  attended  the 
erection  of  continuous  kilns  in  some  localities. 

The  bricks  are  placed  in  the  "  tunnel  "  of  a  kiln,  such  as  the 
one  described,  with  their  longer  sides  parallel  to  the  length  of 
the  kiln,  but  a  few  courses  of  bricks  at  right  angles  to  the 
others  are  valuable  as  "  ties."  The  bricks  are  placed  about 
J  inch  or  |  inch  apart,  so  that  the  gases  may  travel  between 
them  and  heat  them  uniformly.  For  the  same  reason  a  space 
of  one  or  more  inches  is  left  between  each  blade  or  row  of  bricks. 
Immediately  under  each  feed  hole  the  bricks  are  arranged  to 
form  "  fireplaces  "  in  which  the  fuel  can  burn  ;  these  fire- 
places may  take  the  form  of  hollow  shafts  built  of  bricks,  with 
occasional  projecting  bricks  to  prevent  all  the  fuel  from  falling 
directly  to  the  bottom,  or  a  trench  or  space  the  full  width  of 
the  chamber  may  be  left  for  the  same  purpose.  The  former 
method  is  usually  regarded  as  being  the  more  economical,  as 
it  wastes  less  space,  but  as  one  properly  constructed  trench  to 
each  fourteen  feet  of  tunnel  length  is  usually  sufficient,  there 
is  very  little  difference  in  fuel  consumption  between  the  two 
constructions.  By  thus  keeping  the  fuel  entirely  out  of 

1  For  a  description  of  continuous  kilns  in  which  permanent  partitions  are  used, 
see  later  under  the  caption  Chamber  Kilns. 


BURNING   BRICKS 


351 


contact  with  the  bricks  to  be  burned,  the  colour  of  the  latter 
is  greatly  improved,  and  whereas  only  common  bricks  can  be 
burned  in  the  original  Hoffmann  kilns,  facing  bricks  can  be 
burned  in  those  provided  with  troughs  and  grates.  The  use 
of  grates  also  enable  the  kilns  to  be  fired  from  the  front, 
i.e.,  from  the  ground  level  if  this  is  preferred.  As  already 
remarked,  there  are  many  modifications  of  the  Hoffmann 
kiln,  in  all  the  more  important  of  which  trenches,  troughs 
or  grates  for  the  fuel  are  employed  as  in  the  Guthrie  and 
Belgian  kilns. 

Assuming  that  the  kiln  is  in  full  work,  what  takes  place  is, 
approximately,  as  follows  :  the  fuel  is  fed  into  the  feed  holes 
covering  three  chambers  (Nos.  1,  2  and  3)  or  about  forty  feet 

4 Air  travels 

towards 


**WJ  Hot  Gases  tra  vet >  *-Air  enters 

here  and  is  warmed. 

FIG.  107.— The  Round  of  the  Kiln. 

of  tunnel  length,  a  light  charge  of  fuel  being  placed  in  each  hole 
every  quarter  of  an  hour.  It  is  essential  that  the  amount  of 
fuel  used  should  not  be  too  large  ;  sufficient  to  fill  an  ordinary 
quart  jug  is  ample,  though  in  practice  a  very  small  shovel  is 
the  most  convenient  instrument  for  introducing  the  fuel. 
In  a  properly  managed  kiln  the  three  chambers  to  which  fuel 
is  added  will  all  be  at  a  red-heat,  and  No.  1  will  be  nearly 
finished.  The  hot  gases  from  the  burning  fuel  will  be  carried 
by  the  draught  through  the  five  succeeding  chambers  (Nos.  4, 
5,  6,  7  and  8)  and  will  gradually  heat  (i.e.,  pre-heat)  them 
without  their  requiring  any  attention.  After  this,  the  gases 
will  be  of  so  low  a  temperature  that  they  are  no  longer  useful 
and  are  taken  through  the  flue  in  chamber  No.  8  into  the  main 
flue  and  so  to  the  chimney.  All  the  dampers  in  chambers 


352  METHODS   OF   BRICKMAKING 

1  to  7  are  meanwhile  kept  closed,  so  that  all  the  available  heat 
is  used  in  warming  the  bricks  to  be  burned. 

Chambers  Nos.  9,  10  and  11  contain  freshly-set  bricks  and 
these  must  be  separated  from  the  remainder  of  the  kiln  by 
partitions  of  paper  or  metal  running  across  the  whole  of  each 
side  of  the  chamber,  and  their  temperature  must  usually  be 
raised  to  at  least  120°  C.  by  a  separate  supply  of  heat  ;  to 
heat  them  by  waste  gases  would  usually  cause  them  to  be  badly 
scummed  and  so  spoiled,  though  for  some  purposes  this  would 
not  matter,  and  they  may  then  be  taken  at  once  into  what 
is  termed  the  "  round  of  the  kiln  "  without  any  preliminary 
heating.  Ordinarily,  however,  the  bricks  must  be  heated 
by  as  pure  air  as  possible,  until  their  temperature  is  such  that 
no  condensation  products  can  form  upon  them  ;  120°  C.  being 
generally  a  suitable  temperature  for  this  purpose.  The  purest 
warm  air  obtainable  is  that  which  is  drawn  through  the  cham- 
bers containing  cooling  bricks,  and  many  kilns  have  specially 
arranged  flues  for  the  supply  of  warm  air  for  this  purpose. 
Another,  but  less  satisfactory  method  of  warming  the  bricks 
to  120°  C.  consists  in  lighting  a  small  fire  in  the  wicket  and 
allowing  air  to  pass  over  this  into  the  chamber  to  be  warmed. 
During  this  warming  of  the  bricks  the  moisture  present  in 
them  is  driven  off,  and  on  cool  days  it  forms  a  white  smoke, 
whence  this  first  stage  of  the  burning  is  frequently  termed 
water-smoking  or  shortly,  smoking. 

As  soon  as  the  bricks  have  reached  a  temperature  of  about 
120°  C.  the  partition  between  No.  8  and  9  is  removed  (or, 
if  of  paper,  is  torn)  so  as  to  admit  the  hot  gases.  The  damper 
in  No.  8  is  closed,  the  supply  of  warm  air  to  No.  9  is  shut  off 
and  any  opening  made  in  connection  with  the  wicket  fire  is 
closed.  The  hot  gases  from  the  fuel  then  pass  into  No.  9 
chamber  and  the  latter  is  then  said  to  be  "  taken  into  the  round 
of  the  kiln."  Meanwhile,  chamber  No.  12  has  been  filled, 
and  the  "  smoking  "  of  this  chamber  is,  therefore,  commenced 
at  once.  Chamber  No.  13  is,  meanwhile,  empty  or  being 
emptied,  Chambers  14,  15  and  16,  contain  finished  bricks 
which  are  cooling,  this  being  accomplished  automatically  by 
the  draught  of  the  kiln  which  draws  air  through  the  open 
doorway  of  No.  13  through  the  bricks.  The  air  thus  admitted 


BURNING  BRICKS  353 

first  comes  into  contact  with  almost  cool  bricks,  and  becomes 
gradually  hotter  in  its  journey  until,  when  it  reaches  the  burning 
fuel,  it  is  of  the  same  temperature  as  the  hottest  bricks  in  the 
kiln  and  ensures,  with  careful  management,  a  very  complete 
combustion  of  the  fuel  with  scarcely  any  avoidable  waste  of 
heat. 

Any  description  of  the  working  of  a  continuous  kiln  must, 
necessarily,  appear  complicated,  in  reality  these  kilns  are  quite 
simple.  As  soon  as  a  chamber  is  filled,  its  contents  are  first 
warmed  by  hot  air  or  a  wicket  fire,  and  then  it  is  taken  into  the 
round  of  the  kiln  as  described.  It  then  needs  no  further  atten- 
tion until  it  has  become  so  hot  that  a  little  fuel  must  be  fed 
into  it  in  order  to  complete  the  burning.  As  soon  as  the 
contents  of  this  chamber  have  been  heated  sufficiently,  the 
addition  of  coal  to  it  is  stopped,  another  chamber  is  taken 
into  the  round  of  the  kiln,  and  so  on  ;  one  chamber  being 
emptied  and  another  being  filled  continuously,  and  the  fire 
travelling  round  and  round  the  kiln  in  a  perfectly  regular  and 
continuous  manner.  The  work  of  the  firemen  is  much  lighter 
than  for  single  kilns  of  equal  output,  and  so  long  as  the  draught 
created  by  the  chimney  remains  steady  and  the  chambers 
are  filled,  emptied  and  fired  regularly,  there  is  little  or  no 
trouble. 

Unfortunately,  climatic  changes  greatly  affect  the  draught 
and  render  constant  watchfulness  on  the  burner  essential, 
and  even  with  all  the  care  possible,  irregular  heating  will  occur 
in  stormy  weather  so  long  as  a  chimney  is  used  to  create 
the  draught.  For  this  reason,  a  number  of  firms  have 
installed  large  fans — usually  over  six  feet  diameter — and  in 
this  way  obtain  a  more  powerful  and  perfectly  steady  draught. 
When  carefully  managed,  their  use  for  this  purpose  is  highly 
advantageous,  but  like  all  other  machinery,  a  fan  requires 
to  be  understood,  and  well  cared  for,  or  it  may  cause  trouble. 
The  few  failures  which  have  arisen  from  the  use  of  fans 
have,  so  far  as  the  author  has  been  able  to  investigate  them, 
been  due  to  three  causes,  none  of  which  are  the  fault  of  the  fan 
itself  :  (1)  the  use  of  too  small  a  fan,  (2)  careless  or  improper 
management,  and  (3)  failure  to  use  and  follow  the  indications 
of  a  recording  draught  gauge.  A  fan,  being  a  far  more  powerful 

c.  A  A 


354  METHODS   OF   BRICKMAKING 

draught  producer  than  a  chimney,  requires  to  be  kept  under 
proper  control  ;  it  then  works  in  the  most  satisfactory  manner 
possible. 

When  it  is  desired  to  make  bricks  in  large  numbers  over  a 
period  not  exceeding  five  or  six  years — the  colour  of  the  bricks 
being  unimportant  so  long  as  they  are  strong  and  well  shaped 
— a  much  cheaper  kiln  may  be  constructed  by  omitting  the 
arched  roof  and  replacing  it  by  a  platting  or  cover  of  bricks 
laid  flat  and  close  together,  and  covering  them  with  a  3-inch 
or  4-inch  layer  of  cinder  dust,  sand  or  ballast.  Holes  are, 
of  course,  left  in  the  platting  for  the  insertion  of  feed-caps, 
it  being  usually  convenient  to  employ  square  caps  in  this  case. 
In  a  temporary  kiln  of  this  character  the  simplest  possible  form 
of  construction  should  be  used,  the  most  convenient  plan  being 
a  large  rectangle  112  feet  X  18  feet  internally,  with  stout  walls 
well  buttressed  outside.  Eight  openings,  each  wide  enough 
to  admit  a  cart,  are  left  in  each  side  in  order  that  the  kiln  may 
be  filled  and  emptied  expeditiously  ;  these  openings  are  closed 
by  temporary  brickwork,  well  coated  with  clay  paste  when  the 
"  chamber  "  to  which  they  correspond  is  filled  with  fresh 
bricks. 

Instead  of  the  massive  centre  usual  to  permanent  kilns,  a 
single  brickwork  partition  may  be  built  longitudinally  down 
the  centre  of  the  kiln,  and  a  flue  carried  down  each  side  of  the 
kiln  about  one  foot  below  the  ground  level  may  be  connected 
to  suitable  openings  (controlled  by  dampers)  in  the  outer 
walls  of  the  kiln  and  to  the  fan  used  to  create  the  draught. 

Kilns  of  this  archless  type  have  been  used  (with  greater  or 
less  modification)  with  great  success  in  the  colonies  and  in 
the  tropics,  the  saving  in  fuel  as  compared  with  ordinary  kilns 
being  fully  50  per  cent,  and  sometimes  75  per  cent.  Two  such 
kilns  are  in  use  in  Great  Britain  at  the  present  time  under 
licence  from  the  patentee,  H.  Harrison. 

Well-built  Hoffmann  kilns  will  usually  require  2J  to  3  cwt. 
of  coal  per  thousand  bricks  burned,  but  much  depends  on  the 
quality  of  the  coal  used,  on  the  nature  of  the  clay  and  on  the 
amount  of  vitrification  needed  in  the  goods.  If  grates  or 
troughs  and  hot-air  flues  are  used,  the  quantity  of  fuel  will 
rise  to  3  to  4  cwt.  per  thousand  bricks,  but  should  seldom 


BURNING  BRICKS  355 

exceed  the  higher  figure.  The  fuel  consumption  of  most 
Hoffmann  kilns  is,  therefore,  about  half  that  of  single  kilns 
when  a  normal  brick  clay  is  being  burned. 

It  may  here  be  noted  that  the  number  of  patents  which  have 
been  taken  out  for  some  small  modification  of  the  Hoffmann 
kiln  is  very  large.  Consequently,  there  are  many  kilns 
advertised  under  various  distinctive  names  which  are.  in 
reality,  nothing  but  Hoffmann  kilns  with  a  flue  for  supplying 
hot  air  to  the  chambers  to  be  "  smoked,"  and  grates  or  troughs 
for  the  fuel  instead  of  the  original  shafts.  The  distinctive 
features  of  all  these  separate  modifications  of  Hoffmann's 
original  kiln  would  occupy  more  space  than  can  be  devoted 
to  them  in  the  present  volume.  Readers  who  wish  to  study 
the  details  more  freely  will  find  them  set  out  very  fully  in  the 
author's  "  Kilns  and  Kiln  Burning."  Two  modifications  may, 
however,  be  mentioned  here  on  account  of  their  different 
construction  in  several  important  particulars,  and  particularly 
because  they  are  really  composed  of  a  series  of  chambers 
connected  in  such  a  manner  as  to  work  continuously.  For 
this  reason  they  are  conveniently  considered  as  a  different 
type  of  kiln,  and  are  suitably  termed  chamber  kilns. 

The  "  Staffordshire  "  kiln,  in  general  shape,  resembles  the 
modern  Hoffmann  kiln,  but  internally  it  is  divided  into  a 
number  of  chambers  by  permanent  partitions.  These 
partitions  have  vertical  slits  in  them  which  can  be  closed  by 
means  of  vertical  dampers  when  it  is  required  to  shut  off  a 
chamber  for  water-smoking,  annealing  or  other  purposes.  A 
number  of  special  flues  are  also  constructed  in  the  roof  and 
floor  of  the  kiln  so  as  to  provide  an  ample  supply  of  hot  air 
for  smoking,  aiding  combustion  or  for  prolonged  heating  in 
a  current  of  hot  air,  such  as  is  needed  with  certain  brick  clays 
which,  otherwise,  form  black  cores  or  "  hearts."  As  the 
amount  of  air  which  can  be  heated  by  drawing  it  through  the 
chambers  containing  cooling  bricks  is  not  sufficient  for  all 
these  purposes,  the  Staffordshire  kiln  is  provided  with  additional 
flues  in  the  arched  roof  of  the  kiln  through  which  an  additional 
supply  of  air  can  be  drawn  and  heated.  It  should,  however,  be 
pointed  out  that  the  use  of  these  flues  involves  the  consumption 
of  an  additional  amount  of  fuel  in  order  to  replace  the  heat 

AA2 


356  METHODS   OF   BRICKMAKING 

supplied  to  the  air  by  them.  The  convenience  of  this  method 
of  heating  the  air  is,  however,  so  great  that  it  is,  in  the  end,  one 
of  the  most  satisfactory  ways  of  obtaining  a  sufficiently  large 
supply  of  hot  air.  A  number  of  other  kilns  have  accessory 
flues  of  this  character,  thus,  in  Brown's  kiln  the  flues  are 
placed  beneath  the  floor  instead  of  in  the  roof. 

With  some  clays  it  is  necessary  to  provide  for  the  rapid 
removal  of  large  volumes  of  steam  produced  during  the 
"  smoking,"  and  here  again  the  "  Staffordshire  "  kiln  is  well 
prepared.  As  in  some  cases  the  steam  is  best  removed  from 
the  upper  part  of  the  kiln,  and  in  others  from  near  the  floor 
level,  arrangements  for  both  are  provided,  and  the  steam 
evolved  by  the  goods  is  therefore  removed  in  any  direction 
desired  and  at  any  speed  which  may  be  considered  suitable. 
This  kiln  has,  in  fact,  long  been  regarded  as  the  most  completely 
fitted  continuous  kiln  at  present  in  use,  and  in  it  large  numbers 
of  best  facing  bricks,  terra-cotta  and  other  valuable  clay 
products  are  burned  with  complete  success,  the  results  being 
in  every  way  equal  to  those  obtained  by  the  best  single  down- 
draught  kilns  and  this,  notwithstanding  the  fact  that  the 
draught  in  the  Staffordshire  kiln  is  horizontal  rather  than 
"  down-draught  "  in  direction,  though  it  can  be  made  com- 
pletely down-draught  if  required.  The  fuel  consumption  of  a 
Staffordshire  kiln  is  about  J  to  1  cwt.  more  than  that  of  a 
plain  Hoffmann  kiln,  but  as  the  latter  cannot  be  used  for 
facing  bricks  and  terra-cotta,  the  additional  fuel  consumption 
may  be  regarded  as  necessary. 

Where  a  completely  down-draught  kiln  is  required  without 
the  additional  flues  provided  in  the  kiln  just  mentioned  and 
yet  with  the  low  fuel  consumption  of  a  continuous  kiln,  the 
"  Ruabon  "  kiln  will  prove  very  suitable.  This  consists  of  a 
number  of  down-draught  kilns  arranged  consecutively,  the 
waste  gases  from  one  kiln  passing  directly  into  the  next  and 
through  as  many  subsequent  ones  as  may  be  considered 
desirable.  By  arranging  the  kilns  in  this  manner  most  of  the 
advantages  of  a  continuous  kiln  are  obtained  in  combination 
with  the  excellent  colour  and  strength  of  goods  burned  in 
single  down-draught  kilns.  The  Ruabon  kiln  requires  about 
1 J  to  2  cwt.  per  thousand  bricks  more  than  the  plain  Hoffmann 


BURNING   BRICKS  357 

kiln,  or  about  fths  of  that  of  separate  down-draught  kilns 
of  equal  capacity. 

From  what  has  been  stated  about  continuous  and  chamber 
kilns,  the  reader  will  understand  that  the  greatest  advantages 
are  derived  from  those  kilns  in  which  the  gases  from  the  kiln 
travel  the  longest  distance  over  and  among  bricks  before  they 
enter  the  exit  flue,  and  those  in  which  the  distance  the  incoming 
air  has  to  travel  among  cooling  bricks  before  it  reaches  the 
burning  fuel.  There  is  a  great  tendency  in  most  British  works 
to  use  kilns  in  which  these  distances  are  far  too  short,  and 
consequently  the  amount  of  fuel  burned  is  greater  than  would 
be  the  case  if  the  kilns  had  been  more  advantageously  designed , 
For  instance,  in  twelve-chamber  and  fourteen-chamber  kilns 
a  wastage  of  J  to  1  cwt.  of  coal  per  thousand  bricks  is  common, 
and  would  have  been  avoided  had  the  kiln  been  built  with 
sixteen  or  eighteen  chambers. 

Most  continuous  kilns  in  the  United  Kingdom  are  too  wide, 
and  therefore  too  short,  this  defect  having  originated  from  the 
fact  that  it  is  easier  to  get  a  good  draught  with  a  short,  wide 
kiln  than  with  a  long,  narrow  one.  Since  the  substitution  of 
fans  for  chimneys  in  creating  the  draught,  the  use  of  such 
wide  kilns  is  no  longer  necessary  for  bricks,  especially  as 
narrower  kilns  have  several  advantages.  Hence,  about 
thirty  years  ago,  Jacob  Biihrer  effected  a  great  saving  in  fuel 
and  a  large  increase  in  output  by  the  use  of  a  continuous  kiln 
with  a  tunnel  about  twice  as  long  and  half  as  wide  as  those 
commonly  in  use.  In  order  to  overcome  the  difficulties  of 
construction  and  loss  of  heat  incidental  to  an  extremely  long 
and  narrow  kiln,  Biihrer  arranged  his  tunnel  in  a  zigzag 
manner  (Fig.  108)  so  that  whilst  externally  his  kiln  is  square 
in  plan,  its  effective  tunnel  length  is  almost  double  that  of  a 
Hoffmann  kiln  covering  the  same  area.  In  this  manner  a 
continuous  kiln  can  be  built  for  common  bricks  with  a  fuel 
consumption  of  about  2  to  2J  cwt.  per  thousand,  and  as  the 
fire  travels  very  rapidly  forward  in  so  narrow  a  kiln,  the 
conveniences  and  advantages  which  accrue  from  this  are 
readily  obtained.  For  various  reasons  Biihrer's  kilns  have 
made  no  headway  in  the  United  Kingdom,  there  being  only 
one — and  that  a  new  one — in  use  at  the  present  time,  whereas 


358 


METHODS   OF  BRICKMAKING 


in  Europe  and  abroad  generally,  they  have  been  built  in  large 
numbers. 

The  use  of  a  tunnel  kiln  in  which  the  bricks  to  be  burned  are 

placed  on  small  waggons  and 
run  slowly  through  a  tunnel, 
the  centre  of  which  is  heated 
to  the  finishing  temperature 
of  the  bricks,  has  met  with 
considerable  success  abroad, 
but  has  not  been  used  in  the 
United  Kingdom  except  for 
pottery  and  other  wares 
which  are  more  expensive 
than  bricks.  The  cost  of 
repairs  of  the  central  portion 
of  the  tunnel  when  such  solid 
materials  as  bricks  are  being 
burned,  is  greater  than  the 
cost  of  repairs  of  other  types 
of  continuous  kilns. 

Kilns  in  which  gas  is  used 
instead  of  solid  fuel  are 
slowly  increasing  in  popu- 
larity. They  are  no  more 
economical  than  continuous 
kilns  in  which  coal  is  used, 
so  far  as  fuel  consumption  is 
concerned,  but  they  are  easier 
to  regulate  and  to  maintain 
at  a  constant  temperature, 
and  with  some  clays  this  is 
very  important. 

The  changes  which  occur 
to  bricks  and  other  articles 
whilst  they  are  in  the  kilns, 
form  the  subject  of  the  following  chapter.  It  is  here  suffi- 
cient to  remark  that  the  heating  must  be  slow  and  steady 
from  start  to  finish,  as  too  rapid  a  rise  in  temperature  or 
sudden  changes  in  the  temperature  will  result  in  cracked, 


BURNING   BRICKS  359 

warped  or  twisted  goods.  Similarly,  the  cooling  must 
also  be  slow  and  uniform,  great  care  being  taken  to  avoid 
cold  air  impinging  directly  on  to  very  hot  goods.  It  is  also 
obvious  that  the  effect  of  the  heat  must  be  watched  very  care- 
fully ;  if  insufficiently  heated,  the  goods  will  be  weak  and 
porous,  whilst  if  over  burned  they  will  be  misshapen.  The 
man  in  charge  of  the  kiln  ascertains  the  completion  of  the  burn- 
ing- by  means  of  trials,  shrinkage  measurements  or  Seger  cones. 
Pyrometers  are  sometimes  used  to  determine  the  temperature 
actually  reached,  but  what  is  required  is  the  effect  of  the  heat — 
which  is  a  function  of  the  time  of  heating  as  well  as  of  the 
temperature — and  this  cannot  be  ascertained  by  a  pyrometer 
such  as  is  used  in  other  industries. 

Trials  consist  of  bricks  or  other  pieces  of  clay  which  are 
placed  in  such  parts  of  the  kiln  as  to  be  easily  removable 
whilst  the  kiln  is  being  fired.  One  or  more  of  these  trials  is 
withdrawn  at  intervals  and  is  carefully  examined.  If  it 
possesses  all  the  desired  characteristics  of  the  finished  brick 
the  burning  is  considered  to  be  finished  ;  otherwise,  it  is 
continued  until  a  satisfactorily  fired  trial  is  obtained.  In 
most  cases,  the  properties  of  trial  bricks  are  not  sufficiently 
distinctive  to  be  clearly  discernible,  but  with  some  clays, 
trials  are  invaluable.  For  instance,  some  bricks,  when  broken, 
show  a  black  core  or  "  heart,"  due  to  too  rapid  or  incomplete 
burning.  Such  cores  are  objectionable  in  several  ways,  and 
they  can  be  removed  or  prevented  by  careful  heating  with  an 
ample  supply  of  air  at  a  temperature  corresponding  to  a  dull- 
red  heat.  When  a  clay  has  a  tendency  to  produce  bricks  with 
this  defect,  the  withdrawal  of  trials  at  intervals  is  one  of  the 
simplest  and  most  efficacious  methods  of  ascertaining  whether 
the  formation  of  dark  cores  is  being  prevented  or  whether  the 
cores  are  being  properly  burned  out. 

Trials  are  also  useful,  though  to  a  much  smaller  extent, 
when  bricks  of  a  certain  colour  are  required.  Unfortunately, 
the  rapid  cooling  of  the  trials — which  is  unavoidable — has  a 
marked  influence  on  the  colour  and  renders  accurate  comparison 
impossible  with  some  clays. 

Glazed  bricks  are  almost  invariably  burned  with  the  aid  of 
trials,  as  the  appearance  of  the  glaze  can  be  readily  judged  in 


360  METHODS    OF    BRICKMAKING 

this  manner  and  the  firing  regulated  accordingly.  Pieces  of 
clay  of  convenient  shape  are  covered  with  glaze  in  the  same 
manner  as  the  bricks — usually  by  dipping  them  into  a  cream 
or  slurry  made  by  mixing  the  glaze  materials  with  water. 
Several  of  these  trial  pieces  are  placed  in  various  parts  of  the 
kiln  and  are  withdrawn  and  examined  at  intervals.  A  trial 
which  has  once  been  withdrawn  should  never  be  replaced  ; 
hence  the  need  of  a  number  of  trials  in  each  kiln  or  chamber. 

Shrinkage  measurements  form  an  admirable  method  of 
determining  the  effect  of  the  firing.  All  clays  shrink  when 
heated,  and  if  the  conditions  of  manufacture  are  kept  reason- 
ably constant,  the  amount  of  shrinkage  will  be  uniform.  The 
total  shrinkage  which  occurs  in  the  kiln  is  not  great,  averaging 
about  |  inch  per  linear  foot  of  material,  so  that  a  large  amount 
of  material  is  needed  for  an  accurate  measurement.  The 
method  usually  adopted  is  very  ingenious  and  consists  in 
inserting  an  iron  through  the  roof  of  the  kiln  until  it  touches 
the  top  of  the  bricks  being  burned.  As  the  height  to  which  the 
bricks  are  stacked  in  the  kiln  is  known,  their  height  during  and 
after  burning  can  readily  be  ascertained  by  noting  how  far 
the  iron  rod  must  be  inserted.  The  pressure  of  the  bricks 
in  the  upper  part  of  the  kiln  affects  the  contraction  of  the  lower 
ones  to  a  slight  extent,  so  that  the  shrinkage  measured  in  this 
manner  is  somewhat  greater  than  the  average  shrinkage  of 
each  brick.  It  will  usually  be  found  that  the  iron  "  shrinkage 
rod  "  can  be  inserted  about  three  to  five  inches  further  into  the 
kiln  after  the  burning  is  complete,  than  it  can  at  the  start  ; 
the  "  settlement  "  of  bricks  in  different  districts  varies,  however, 
and  in  some  localities  a  shrinkage  of  nine  or  even  ten  inches 
is  not  unusual.  Bricks  fired  in  clamps  with  a  layer  of  fuel 
between  the  courses  appear  to  shrink  much  more  than  those 
burned  in  kilns,  because,  as  the  fuel  burns  away,  the  bricks  sink 
and  take  its  place. 

Measurements  of  the  shrinkage  of  trial  pieces  are  less  satis- 
factory, as  the  roughness  of  the  surface  makes  the  errors  of 
measurement  on  the  smaller  pieces  very  great. 

Seger  cones  are  small  pyramids  made  of  special  materials. 
When  placed  in  a  kiln  these  pyramids  bend  over  and  form  an 
arch,  the  heat-effect  corresponding  to  a  particular  cone  being 


BURNING   BRICKS  361 

quite  constant.  The  cones  are  numbered  and  form  a  very 
extensive  series,  those  most  used  for  bricks  being  :— 

No.  015a  to  No.  2a  for  building  bricks. 

No.  la  to  14  for  sintered  bricks,  paviours  and  clinkers. 

No.  6a  to  9  for  glazed  bricks. 

No.  6  to  20  for  fire-bricks. 

Strictly  speaking,  the  Seger  cones  do  not  correspond  to  any 
definite  temperatures,  but  to  heat  effects  on  the  materials 
of  which  they  are  made.  For  this  reason  they  are  more  valu- 
able than  simple  temperature  indicators,  such  as  pyrometers. 
If,  however,  the  heating  is  steady  and  the  rise  in  temperature 
corresponds  to  that  at  which  the  cones  have  been  tested  by  the 
manufacturers — about  |°  C.  per  minute — the  cones  correspond 
with  sufficient  accuracy  to  the  tables  of  temperature  supplied 
with  them.  For  slower  or  more  rapid  heating,  an  error, 
depending  on  the  time  of  heating,  is  introduced  ;  this  does  not 
interfere  with  the  use  of  cones  in  kilns,  as  in  the  latter  the  rate 
of  heating  should  be  as  constant  as  possible  each  time  the  kiln 
is  burned,  and  cones  can,  therefore,  be  relied  upon  to  indicate 
whether  the  bricks  and  other  goods  have  been  sufficiently 
heated. 

The  use  of  these  cones  is  quite  simple  :  a  trial  is  first  made  to 
ascertain  which  cone  corresponds  to  the  correct  finishing  point 
of  the  kiln,  and  afterwards  the  cones  are  arranged  in  groups  of 
three  in  various  parts  of  the  kiln.  In  each  group  one  cone 
bends  at  about  20°  C.  below  the  correct  finishing  point  of  the 
kiln,  the  second  cone  indicates  this  finishing  point,  and  the 
third,  which  should  bend  about  20°  C.  above  the  second,  is 
used  as  a  precaution  to  show  that  the  kiln  has  not  been  over- 
heated. The  cones  are  placed  so  that  they  can  be  viewed 
through  spy-holes  in  the  walls  or  top  of  the  kiln,  and  by  their 
use  the  progress  of  the  burning  can  be  watched  with  the  greatest 
ease.  If  a  clay  is  difficult  to  manage  at  a  temperature  much 
below  the  finishing  point,  the  use  of  additional  cones  corre- 
sponding to  the  lower  temperature  enables  the  burner  to  know 
when  any  desired  stage  of  the  burning  above  the  darkest 
visible  red  heat  has  been  reached. 

A  wise  burner  will  not  rely  solely  on  trials  or  shrinkage  or 
cones,  but  will  employ  all  three  methods  of  controlling  the 


362  METHODS    OF    BRICKMAKING 

kiln,  and  will,  in  addition,  be  able  to  tell  by  the  colour  of  the 
inside  of  the  kiln  and  by  its  general  appearance  whether  the 
burning  is  or  is  not  proceeding  satisfactorily. 

The  manufacture  of  bricks  by  machinery  is  not  difficult  so 
far  as  the  production  of  the  undried  and  unburned  bricks  is 
concerned — though  even  here  more  technical  skill  and  experience 
are  necessary  than  is  generally  imagined.  The  chief  difficulties 
of  manufacture  occur  in  the  drying  and  in  the  burning,  and 
so  complex  are  the  changes  which  occur  in  these  two  stages 
of  manufacture  that  even  when  the  greatest  care  is  taken  by 
the  most  skilful  expert  an  occasional  batch  of  defective  bricks 
is  produced. 

If  a  firm  manufacturing  facing  bricks  can  continue  week  in 
week  out  for  several  years  without  producing  more  than 
4  per  cent,  of  their  output  which  is  unsaleable  as  facing  bricks, 
such  a  firm  may  be  considered  very  fortunate.  It  is  difficult 
to  state  the  average  ratio  of  facing  bricks  actually  obtained 
to  those  made,  but  it  will  seldom  exceed  90  per  cent.,  most  of 
the  remainder  being  sold  as  common  bricks,  and  a  few  being 
quite  unsaleable  except  as  brickbats.  In  the  manufacture  of 
common  building  bricks  the  ratio  of  saleable  bricks  to  bricks 
made  is  about  97  per  cent,  in  well-managed  works,  but  with 
inferior  management  or  unskilful  burners  it  may  drop,  at 
times,  to  as  low  as  50  per  cent. 

In  the  manufacture  of  firebricks,  which  are  usually  moulded 
by  hand,  dried  on  a  steam-heated  floor  and  burned  at  cones 
6  to  20,  according  to  the  quality  of  the  material  used,  the 
proportion  of  first-class  bricks  to  the  total  output  is  about 
the  same  as  that  for  facing  bricks.  Some  firms  are,  however, 
more  careful  in  sorting,  and  in  this  way  they  are  able  to  supply 
bricks  for  various  purposes  at  prices  which  represent  a  loss 
of  not  more  than  Ij  to  2  per  cent,  of  the  total  number  of 
bricks  made. 


CHAPTER  XIII 

THE  CHEMICAL  AND  OTHER  CHANGES  IN  DRYING  AND  BURNING 

BRICKS 

IT  has  been  explained  in  a  previous  chapter  that  the  "  clays  " 
used  for  the  manufacture  of  building  bricks  are  far  from  pure. 
They  may,  indeed,  be  regarded  as  mixtures  of  (a)  true  clay, 
(b)  sand,  and  (c)  fusible  minerals. 

In  the  green  or  unburned  bricks  these  materials  are  in  the 
form  of  a  compressed  powder  containing  grains  of  widely 
different  sizes,  from  the  coarse  pieces  of  stone  or  gravel  of 
J  inch  diameter  to  the  particles  of  clay  which  are  so  fine  that 
their  shape  cannot  be  clearly  distinguished  under  the  most 
powerful  microscope.  In  addition  to  these  materials,  most 
raw  bricks  contain  a  certain  amount  of  water  which  has  been 
added  to  secure  the  proper  cohesion  of  the  solid  particles. 
This  water,  which  in  plastic  bricks  may  easily  amount  to 
1  Ib.  in  each  brick  made,  must  be  completely  removed  during 
the  drying. 

In  the  freshly-made  brick,  each  solid  particle  may  be  regarded 
as  surrounded  more  or  less  perfectly  by  a  film  of  water  which 
keeps  each  of  the  solid  particles  slightly  separated  from  each 
other.  When  a  brick  is  dried,  however,  this  water  is  removed, 
and  consequently  the  solid  particles  move  nearer  together. 
This  is  shown  diagrammatically  in  Fig.  109,  in  which  the  solid 
particles  are  represented  by  black  discs,  the  space  between 
them  in  the  left  side  (A)  of  the  illustration  indicating  the 
water  which  separates  them.  When  this  water  has  been 
removed  by  drying,  the  particles  take  up  the  position  shown 
in  the  right  side  (B)  of  the  diagram,  and  it  will  at  once  be 
observed  that  the  total  volume  of  the  material  has  been  reduced 
by  the  removal  of  the  water.  In  other  words  the  brick  has 
shrunk  on  drying. 


364    CHANGES  IN  DRYING  AND  BURNING  BRICKS 


Whether  the  water  in  the  freshly-made  bricks  is  really  in 
the  form  of  a  film,  as  suggested  above,  or  whether  it  is  in  a 
state  similar  to  that  contained  in  glue  which  has  been  soaked 
for  several  hours,  is  a  matter  concerning  which  there  is  a 
divergence  of  opinion.  Those  who  regard  clays  as  colloidal 
substances  naturally  maintain  that  the  bulk  of  the  water  is 
retained  within  the  "  meshes "  of  the  colloidal  aggregate, 
whilst  others  hold  that  the  water  which  causes  the  shrinkage 
is  almost,  if  not  entirely,  superficial,  and  acts  as  a  film  surround- 
ing each  particle. 

In  all  probability  the  water  occurs  both  as  a  film  surrounding 
some  of  the  particles  and  also  in  an  adsorbed  or  enmeshed 
condition.  Such  water  is  conveniently  termed  shrinkage  water. 


FIG.  109. 


B 


There  is,  however,  a  small  proportion  of  water  which  occupies 
the  pores  or  spaces  between  the  particles,  even  when  the  latter 
are  packed  as  closely  as  possible  ;  this  is  conveniently  termed 
the  pore  water. 

If  the  diagram  B  is  observed  closely  it  will  be  noticed  that 
there  are  a  number  of  white  spaces  still  left  between  the  black 
discs.  These  contain  the  "  pore  water  "  just  mentioned,  and 
correspond  to  a  stage  in  the  drying  at  which  the  particles 
cannot  move  closer  together,  and  yet  some  water  is  still 
present  in  the  brick  ;  that  is  to  say,  the  shrinkage  ceases 
before  all  the  water  has  been  evaporated.  This  stage  is  very 
important  to  brick  and  terra-cotta  manufacturers,  though  less 
attention  is  paid  to  it  than  its  importance  deserves.  During 
the  first  stage  of  drying,  when  the  particles  are  in  motion,  on 


CHANGES  IN  DRYING  BRICKS  365 

account  of  the  evaporation  of  the  water  which  separates  them 
from  each  other,  there  is  a  great  danger  of  cracking  and 
twisting.  Some  bricks  are,  indeed,  so  sensitive  that  they 
present  great  difficulties,  and  the  clays  from  which  they  are 
made  are  practically  worthless,  because  of  the  large  proportion 
of  cracked  and  warped  bricks  produced.  As  soon  as  the 
shrinkage  ceases,  however,  the  remainder  of  the  water  present 
may  be  readily  evaporated,  the  sensitiveness  of  the  material 
having  almost  entirely  disappeared.  Thus,  bricks  which  have 
to  be  kept  covered  and  fully  protected  from  heat  and  draughts 
during  the  first  stage  of  drying  may  be  placed  on  a  hot  floor 
or  even  on  an  iron  plate  heated  by  a  gas  burner  without  damage 
during  the  removal  of  the  remainder  of  the  water.  The  wise 
manufacturer,  therefore,  ascertains  as  accurately  as  possible 
when  the  shrinkage  during  drying  has  ceased,  and  he  then 
completes  the  removal  of  the  water  at  a  more  rapid  rate, 
usually  by  sending  the  bricks  to.  the  kiln. 

To  remove  the  whole  of  the  water  from  the  bricks  in  a  dryer 
is  to  work  more  slowly  and  less  economically  than  when  all 
the  shrinkage  water  is  taken  out  in  a  dryer  and  the  remainder 
of  the  water  is  removed  in  the  kiln. 

Previous  to  the  bricks  entering  the  kiln,  the  only  changes 
which  occur  in  them  are  the  removal  of  the  water  used  to 
facilitate  the  shaping  of  the  clay  and  the  reduction  in  the  size 
of  the  bricks  due  to  the  particles  drawing  closer  to  each  other. 
Bricks  made  by  the  semi-dry  process  contain  so  little  water 
that  they  are  sent  direct  to  the  kilns,  as  the  drying  shrinkage 
they  undergo  is  practically  negligible.  Bricks  made  of  softer 
and  more  plastic  clay  require  to  be  dried  in  hacks  (p.  335),  on 
a  steam-heated  floor  or  in  a  special  dryer  before  they  can  be 
sent  to  the  kilns.  In  short,  the  changes  in  drying  are 
purely  of  a  physical  character,  unless  it  may  be  assumed  that 
the  clay  is  a  swollen  colloid,  the  water  enmeshed  in  it 
being  in  the  form  of  a  loosely  combined  compound  and  not 
simply  in  an  adsorbed  form.  This  loose  combination  is  quite 
possible  and  has  some  evidence  in  its  favour,  but  the  view 
that  the  clay  simply  changes  physically  during  drying  is  more 
readily  understood,  and  as  it  explains  most,  if  not  all, 
the  phenomena  observed  during  drying,  it  is  generally 


366    CHANGES  IN  DRYING  AND  BURNING  BRICKS 

accepted,  notwithstanding  the  evidence  in  favour  of  the  other 
theory. 

It  is  after  the  bricks  have  entered  the  kilns  that  the  greatest 
and  most  important  changes  occur.  Before  doing  so,  the  bricks 
are  soft,  friable,  grey  masses,  which  must  be  handled  with 
great  care  or  corners  and  arrises  will  be  broken  off.  If  placed 
in  a  bowl  of  water  such  bricks  would  soften  and,  in  time,  would 
fall  to  powder  or  to  a  sticky  shapeless  mass  of  loosely  coherent 
mud.  When  removed  from  the  kiln,  the  bricks  have  increased 
enormously  in  hardness  ;  instead  of  being  weak  and  friable 
their  resistance  to  crushing  is  exceptionally  great,  and  is  equal 


FIG.  110. — Bricks  arranged  in  Hack. 

or  even  superior  to  that  of  stone  ;  they  are  unaffected  by  water 
and  atmospheric  conditions,  and  form  one  of  the  most  durable 
materials  known.  What,  then,  are  the  causes  which  have 
brought  about  so  remarkable  a  change  in  the  nature  of  the 
materials  ?  How  is  it  that  such  changes  can  occur  as  the 
result  of  heating  the  bricks  ? 

In  the  first  place,  these  great  changes  in  the  physical 
properties  of  the  bricks  are  the  result  of  a  series  of  chemical 
changes  of  considerable  complexity.  As  soon  as  the  bricks 
are  warmed  to  a  slightly  higher  temperature  to  that  to  which 
they  have  previously  been  exposed,  the  remainder  of  the 


CHANGES  IN  BURNING  BRICKS  367 

moisture  or  pore  water  in  them  is  evaporated.  This  may  be 
completely  removed  at  or  below  a  temperature  of  120°  C., 
i.e.,  during  the  "  smoking  "  period. 

As  the  temperature  rises  above  120°  C.  the  clay  and  some  of 
the  fusible  minerals  begin  to  decompose,  and  at  500°  C.  the 
decomposition  is  sufficiently  rapid  for  the  steam  produced  to 
become  noticeable.  No  matter  how  carefully  a  clay  is  dried, 
it  will  be  found  on  heating  it  to  a  temperature  of  500  to  600°  C. 
to  evolve  a  considerable  amount  of  water.  This  water  is  not 
added  to  the  clay,  for  it  is  an  essential  part  of  its  constitution  ; 
indeed,  it  does  not  exist  as  water  in  the  clay,  but  the  elements 
of  which  the  clay  molecule  are  composed  are  so  arranged  that 
on  reaching  the  temperature  mentioned  the  clay  is  decomposed, 
and  one  of  the  products  of  its  decomposition  is  water.  The 
amount  of  this  combined  water  in  a  brick  depends  on  the  amount 
of  true  clay  present  ;  ordinarily  it  is  not  more  than  5  or  6  per 
cent,  of  the  dried  brick,  but  in  a  pure  clay,  such  as  the  finest 
grades  of  china  clay,  it  reaches  13  per  cent. 

With  a  steadily  increasing  temperature  the  decomposition 
of  the  clay  and  other  hydrous  minerals  continues,  water  being 
evolved  in  the  form  of  steam,  rapidly  at  500  to  600°  C.,  and  more 
slowly  at  higher  temperatures  as  the  amount  of  undecomposed 
clay  diminishes. 

As  soon  as  the  whole  of  the  clay  has  been  decomposed,  the 
brick  consists  of  a  light  porous  mixture  of  aluminosilicic 
anhydride  (i.e.,  the  calcined  or  dehydrated  clay),1  sand  and 
dehydrated  fusible  minerals,  with  negligible  proportions  of 
hydrous  minerals  which  have  not  yet  been  affected  by  the 
heat.  If  removed  from  the  kiln  at  this  stage  the  bricks  would 
be  weaker  and  more  friable  than  at  first,  but  a  little  harder 
so  far  as  the  average  particles  are  concerned.  The  colour  would 
be  a  dirty,  grey,  with  irregular  patches  of  black  sooty  matter 
derived  from  the  vegetable  matter  contained  in  all  clays. 

As  the  temperature  increases  still  more  and  reaches  a  red 
heat,  the  vegetable  and  other  organic  matter  present  begins 
to  burn,  and  in  some  cases  its  combustion  may  require  special 

1  The  precise  constitution  of  this  material  is  so  complex  that  it  cannot  be 
described  fully  here.  Some  indications  of  its  nature  have  been  given  on 
pp.  40-43. 


368    CHANGES  IN  DRYING  AND  BURNING  BRICKS 

attention.  For  instance,  some  of  the  shales  used  for  brick- 
making  contain  considerable  quantities  of  paraffin  and  allied 
substances  which,  on  heating,  form  readily  combustible  shale 
oil,  and  if  carelessly  managed  this  may  burn  too  rapidly  and 
result  in  the  bricks  being  seriously  overheated  and  so  spoiled. 
In  most  brick  clays,  however,  the  proportion  of  combustible 
matter  is  too  small  to  be  serious  in  its  consequences,  though 
it  is  a  prolific  source  of  blue-black  cores  and  hearts.  In 
any  case  the  kiln  must  be  kept  at  a  dull  red  heat  until  the 
whole  of  this  combustible  matter  has  been  burned  away,  the 
supply  of  fuel  being  restricted  and  that  of  the  air  being 
controlled  so  as  to  secure  complete  combustion  without  an 
undue  rise  in  temperature.  If  at  this  stage  of  the  burning 
the  temperature  rises  too  high,  vitrification  will  set  in  and  it 
will  be  impossible  to  burn  out  the  combustible  matter. 

Between  the  end  of  the  removal  of  the  shrinkage  water  in 
drying  and  the  complete  decomposition  of  the  clay  and  the 
combustion  of  the  organic  matter  present,  the  bricks  undergo 
a  second  contraction  or  kiln  shrinkage  which  is  usually  about 
equal  in  amount  to  (though  frequently  less  than)  that  of  the 
shrinkage  in  drying.  This  kiln  shrinkage  is  partly  due  to  the 
decomposition  of  the  clay  and  other  hydrous  minerals  present, 
whereby  the  particles  become  still  smaller,  and  therefore  occupy 
still  less  space  than  before.  The  kiln  shrinkage  is  also  due,  in 
part,  to  fusion  of  some  of  the  particles,  as  described  later. 
Both  these  forms  of  kiln  shrinkage  are  shown  diagrammatically 
in  Fig.  1 11,  in  which  B  is  reproduced  from  Fig.  109  and  indicates 
the  particles  which  have  shrunk  in  drying,  whilst  C  shows 
further  contraction  which  occurs  in  the  kiln,  due  to  decomposi- 
tion of  the  clay,  and  D  the  material  after  partial  fusion,  the 
fused  portion  having  filled  some  of  the  pore  spaces,  as  shown. 

The  kiln  shrinkage,  which  commences  in  the  manner 
described,  continues  until  the  heating  is  finished,  because  as  the 
temperature  increases  still  further  the  more  fusible  materials 
begin  to  melt  and  then  flow  into  the  pores  or  interstices  between 
the  particles.  This  fusion  increases  as  the  temperature  rises 
or  the  heating  is  prolonged. 

Grains  of  sand,  and  particles  of  stone,  gravel  and  various 
minerals  other  than  clay,  do  not  show  the  same  kind  of  shrink- 


CHANGES  IN  BURNING  BRICKS 


369 


age  as  the  clay  grains.     Many  minerals  only  shrink  when  they 

reach  a  temperature  near  to  the  finishing  point  of  the  firing,  and 

a  few  retain  their  volume  throughout.     The  chief  shrinkage 

occurs    in    the    case   of  limestone 

which,  on  heating,  is  decomposed 

into    carbonic     acid     gas,    which 

escapes,      and     quicklime     which 

remains  in  the  bricks.     All  forms 

of  limestone  are  objectionable  as  a 

constituent    of    bricks,    but    their 

presence  is  particularly  detrimental 

when  they   occur    in    the  form  of 

race    or    septaria     nodules.       The 

quicklime  formed  in  the  kiln  slakes 

slowly  when  the  bricks  are  exposed 

to  the  air,  and  swells  as  it  absorbs 

atmospheric    moisture    and   acids. 

The  white  patches  of  lime  near  the 

surface    of    the    bricks     are    soon 

washed   out  and  leave  unpleasant 

cavities  ;     those    somewhat    below 

the  surface  will    create    strains  in 

the  bricks,  which  may  cause  parts 

of  the  latter  to  crack  or  spall  off, 

and  in  very  bad  cases  the  whole  of 

the  bricks  may  be  reduced  to   a 

mass  of  useless  material.      These 

phenomena  are  known  technically 

as  blowing,    and    are,    as   already 

stated,  due  to  the  lime  present  in 

the  clay.     If   the   lime   compound 

is  in  a   very  finely  divided  state, 

as    in    washed    chalk,   the    bricks 

will  not  blow  ;  indeed,  this  defect  may  sometimes  be  prevented 

by  grinding  the  clay  (and  consequently  the  lime  compound)  to 

an  exceedingly  fine  powder. 

Very  small  grains  of  lime,  soda  or  potash  combine  with  the 
clay  at  high  temperatures  and  form  a  fused,  glassy  material. 
If  the  proportion  of  this  is  sufficiently  large  the  bricks 

c.  B  B 


FIG.  111. 


370    CHANGES  IN  DRYING  AND  BURNING  BRICKS 

will   lose    their    shape    and    may    even    fuse   to    a    shapeless 
slag. 

Iron  compounds,  when  heated  to  a  temperature  at  which  all 
shrinkage  in  the  bricks  ceases,  will  produce  the  red  compound 
which  is  the  characteristic  colouring  material  of  red  bricks, 
unless  the  presence  of  Lime  or  reducing  gases  prevents  the 
formation  of  this  compound. 

Pyrites,  or  iron  disulphide,  on  the  contrary,  produces  black 
slag-like  spots  of  a  ferrous  silicate  or  ferrous  alumino-silicate, 
and  only  under  abnormal  conditions  does  it  form  a  red 
compound. 

Soluble  salts  in  the  clay  will  be  brought  to  the  surface  by 
the  moisture,  as  it  evaporates  during  the  drying  and  early 
stage  of  burning  and  will  form  an  efflorescence  or  scum  on 
the  bricks.  If  the  temperature  is  sufficiently  high  these  salts 
will  combine  with  the  clay  and  will  then  form  a  slight  coating 
of  glaze,  the  scum  simultaneously  disappearing.  The  produc- 
tion of  this  scum  may  be  prevented,  in  many  instances,  by 
rendering  the  salts  insoluble,  as  by  the  addition  of  barium 
carbonate  to  the  raw  clay. 

Minerals  of  the  felspar  type  fuse  independently  of  the  clay  if 
the  temperature  reached  in  the  kiln  is  sufficiently  high,  but 
in  the  manufacture  of  most  building  bricks  it  is  seldom  more 
than  a  very  slight  fusion  of  these  minerals  occurs  except  in  the 
presence  of  some  other  more  fusible  minerals  which,  by  their 
chemical  action,  lower  the  temperature  required  for  the  fusion 
of  the  felspar,  etc. 

If  the  bricks  were  to  be  removed  from  the  kiln  very  soon 
after  the  first  signs  of  fusion  of  some  of  the  particles  had 
commenced,  they  would  be  of  moderate  strength  and  only 
useful  for  inside  work  ;  their  colour  would  be  pale  or  irregular, 
and  they  would  be  typical  baked  bricks. 

Such  bricks  are  used  in  large  quantities  for  the  interiors  of 
buildings  or  where  they  can  be  covered  with  stucco  or  other 
waterproof  material.  The  white  bricks  of  Suffolk,  the  primrose- 
coloured  bricks  of  other  localities,  "  rubbers,"  "  cutters," 
bath-bricks  and  all  firebricks  are  of  this  type,  as  are  many 
red  building  bricks.  They  are  readily  distinguished  from  more 
fully-burned  bricks  by  the  dull  sound  produced  when  two  baked 


COLOUR  OF  BRICKS  371 

bricks  are  struck  together — a  sound  very  different  from  the 
clear,  metallic  note  of  a  fully  vitrified  brick. 

Another  characteristic  of  baked  bricks  is  their  high  porosity, 
whereby  they  can  absorb  upwards  of  15  per  cent,  of  water  and 
yet  appear  to  be  perfectly  dry.  Bricklayers  prefer  such  bricks, 
as  the  mortar  adheres  to  them  more  rapidly,  with  little  or  no 
liability  to  "  float." 

In  the  manufacture  of  red  facing  bricks,  for  building 
purposes,  and  terra-cotta,  the  colour  of  the  burned  material 
is  of  great  importance,  and  as  soon  as  the  articles  have  attained 
sufficient  strength,  the  manufacturer  concentrates  all  his 
attention  in  obtaining  a  pleasant  colour.  For  this  purpose 
he  may  prolong  the  heating  indefinitely  after  vitrification  or 
the  melting  of  the  more  readily  fusible  particles  has  commenced, 
and  will  stop  the  heating  as  soon  as  a  satisfactory  colour  has 
been  obtained. 

The  red  colour  of  bricks  and  terra-cotta  is  primarily  due  to 
iron  compounds  in  the  clay,  these  forming  iron  oxide  or  other 
polymerised  compounds  of  a  red  colour  when  the  clay  is 
heated.  As  the  temperature  in  the  kiln  rises,  or  the  heating 
is  prolonged,  the  colour  increases  in  intensity  until  a  maximum 
is  reached,  after  which  the  fused  material  present  affects  the 
colour,  converting  it  into  a  dark  and  unpleasant  brown  shade 
if  the  atmosphere  inside  the  kiln  is  oxidising,  or  into  a  bluish 
grey  or  black  if  reducing  gases  are  present  in  the  kiln.  Even- 
tually, if  the  temperature  rises  still  further,  or  the  heating  is 
excessively  prolonged,  the  amount  of  fused  matter  produced 
destroys  the  strength  of  the  remainder  and  the  brick  becomes 
a  shapeless  mass  of  slag.  The  burner  must,  therefore,  stop 
the  heating  of  the  kiln  as  soon  as  the  contents  have  reached 
the  desired  colour,  or  even  slightly  before  this,  because  of  the 
effect  of  the  heat  in  the  kiln  whilst  the  fires  are  dying  down. 

When  dark-coloured  bricks  are  required,  the  addition  of 
2  to  4  per  cent,  of  manganese  dioxide  will  often  be  found  to 
produce  the  desired  tint.  When  purple  and  irregular  tones 
are  desired  the  clay  must  be  specially  selected  and  must  be 
fired  with  too  little  and  too  much  air  alternately,  i.e.,  the 
atmosphere  inside  the  kilns  must  be  alternately  oxidising  and 
reducing.  The  tints  produced  on  burning  some  clays  are  a 

B  B  2 


372     CHANGES  IN  DRYING  AND  BURNING  BRICKS 

special  characteristic  of  the  materials  from  certain  loaclities 
and  cannot  be  obtained  artificially  with  other  clays. 

Insufficient  heating  will  produce  bricks  of  too  pale  a  colour  ; 
excessive  heating,  on  the  contrary,  will  result  in  bricks  of  an 
unpleasant  tint.  Where  a  clay  can  be  found  which  can  be 
made  into  bricks  possessing  all  the  desired  mechanical 
properties,  such  as  strength,  lightness,  porosity,  in  addition 
to  an  excellent  colour,  it  may  be  said  to  form  the  ideal  brick 
earth.  Usually,  however,  some  property  must,  in  part,  be 
sacrificed  in  favour  of  the  others.  Thus,  many  bricks  in  the 
Midlands  and  North  of  England  are  heated  to  a  temperature 
which  is  rather  too  high  for  the  production  of  the  most  pleasing 
colour,  but  the  added  strength  gained  by  such  treatment  is 
considered  to  more  than  compensate  this.  It  is  always  a 
temptation  to  the  manufacturer  of  red  bricks  to  pay  the  first 
attention  to  colour,  as  it  is  this  property  which  attracts  the 
attention  of  architects,  though  the  strength  of  the  bricks  is, 
in  most  cases,  of  much  greater  importance. 

Some  very  pretty  colour  effects  are  obtained  by  the  use  of 
reducing  gases  ;  many  of  them  being  largely  of  the  nature  of 
"  accidents."  Thus,  the  admixture  of  a  little  coal  or  coke  dust 
with  the  clay  before  it  is  made  into  bricks  will  often  produce 
pleasing  purple  tones  in  the  bricks.  Sometimes  organic  matter 
and  pyrites  are  present  in  sufficient  quantities  to  produce 
these  plays  of  colour  without  any  effort  on  the  part  of  the 
burner,  but  more  frequently  they  are  produced,  as  suggested, 
by  the  deliberate  addition  of  organic  matter  or  by  purposely 
working  with  a  very  smoky  fire  towards  the  end  of  the 
"  baking  "  stage. 

In  the  manufacture  of  "  blue  bricks  "  in  Staffordshire  and 
elsewhere,  the  material  used  is  so  rich  in  iron  oxide  and  in 
fusible  matter  that  when  smoky  or  reducing  fires  are  used  the 
iron  becomes  reduced  and  forms  a  ferrous  silicate  or  ferrous 
alumino- silicate  of  the  desired  "  blue  "  colour.  The  nature  of 
the  material  is  such  that  when  this  colouring  substance  has 
once  been  formed  it  is  "  fixed  "  by  the  fusible  matter  present, 
and  so  is  not  re-oxidised  during  the  remainder  of  the  heating. 
In  clays  which  are  not  suitable  for  the  manufacture  of  blue 
bricks,  on  the  contrary,  there  is  a  great  tendency  for  the 


FINAL  STAGE  IN  BURNING  BRICKS  373 

bluish  silicate  to  be  re-oxidised  into  the  red  compound  to 
which  red  bricks  owe  their  colour.  Indeed,  if  a  clay  which  is 
ordinarily  used  for  blue  bricks  is  fired  under  purely  oxidising 
conditions,  excellent  red  bricks  may  be  produced. 

Another  curious  fact  about  the  colour  of  bricks  is  that  if 
15  per  cent,  of  chalk  is  mixed  with  a  red-burning  clay,  the 
bricks  withdrawn  from  the  kiln  at  a  temperature  below  about 
750°  C.  will  be  red,  but  those  heated  to  a  higher  temperature 
will  be  white.  A  similar  result  will  be  obtained  if  a  clay 
naturally  containing  both  chalk  and  iron  oxide  is  used,  the 
chalk,  iron  and  clay  uniting  to  form  a  white  alumino- silicate. 

Engineering  bricks  and' others,  in  which  the  greatest  possible 
strength  is  required,  must  be  heated  to  such  an  extent  that  the 
fused  material  in  them  fills  all  the  pores  and  unites  the 
remaining  particles  into  a  very  strong  whole.  Such  bricks 
can  only  be  produced  by  prolonged  heating  at  sufficiently 
high  temperatures,  but  care  is  required  not  to  let  the  partial 
fusion,  or  vitrification,  proceed  too  rapidly  lest  the  bricks 
should  lose  their  shape.  The  metallic  oxides  present  in  the 
clay  are  of  great  importance  in  this  respect,  as  a  clay  containing 
magnesia  will  lose  its  shape  far  less  readily  with  an  equal 
amount  of  vitrification  than  will  one  containing  an  equivalent 
proportion  of  lime  or  alkalies. 

The  final  stage  of  burning  consists,  therefore,  in  raising  the 
temperature  slowly  and  steadily  until  a  sufficient  amount  of 
fused  matter  has  been  produced  to  give  the  bricks  the  necessary 
strength  when  cold.  The  amount  of  this  fusion  or  vitrification 
is  usually  ascertained  by  drawing  trials,  breaking  them  and 
examining  the  texture  of  the  fractured  surface.  Two  pieces 
of  clay  which  have  been  fired  until  vitrified  will  also  ring  with 
a  metallic  note  when  struck  together,  the  dullness  of  the  tone 
being  some  guide  to  the  insufficiency  of  the  heating. 

Where  the  heating  is  pushed  to  the  fullest  extent  possible 
without  loss  of  shape,  the  material  will  be  found  to  resemble  an 
opaque  glass  or  slag  in  character.  It  will  be  excessively  hard, 
entirely  impervious  to  water  and  highly  resistant  to  corrosive 
acids.  Its  colour  will  be  dark,  approaching  a  brownish  black, 
a  slag  grey,  or  what  is  known  technically  as  a  clinker  or 
Staffordshire  blue,  and  its  density  will  be  appreciably  increased. 


374    CHANGES  IN  DRYING  AND  BURNING  BRICKS 

In  size,  articles  which  have  been  burned  to  complete  vitrifica- 
tion will  be  somewhat  less  than  those  which  have  been  merely 
baked,  but  the.  difference  is  not  very  great.  The  formation  of 
gases  in  the  interior  of  the  material  tends  to  cause  swelling  ; 
this,  which  may  prove  a  serious  defect,  may  be  recognised 
by  the  shape  of  the  pores  observable  on  the  fractured  surface 
of  the  article  when  broken. 

The  most  perfect  vitrified  texture  is  that  obtained  in  hard- 
fired  porcelains,  the  raw  materials  for  which  are  more  carefully 
prepared  than  would  be  remunerative  for  bricks. 

It  is  frequently  stated  in  books  and  lectures  on  building 
materials  (obviously  by  those  who  have  little  or  no  real  know- 
ledge of  brickburning)  that  heating  to  one  temperature  will 
produce  red  bricks  and  to  another  temperature  will  produce 
blue  ones.  This  is  quite  erroneous,  as  the  difference  in  colour 
does  not  depend  on  temperature,  but  on  the  nature  of  the 
atmosphere  inside  the  kiln.  Moreover,  clays  which  contain  a 
considerable  proportion  of  fluxing  materials,  such  as  chalk, 
limestone,  magnesia,  soda  and  potash  compounds,  will  fuse 
and  form  a  shapeless  mass  at  temperatures  far  below  those 
used  for  burning  other  brick  clays. 

The  temperature  at  which  a  sufficiently  intense  colour  is 
developed  varies  greatly  with  different  materials  ;  in  some 
bricks  it  is  reached  at  a  temperature  of  about  800°  C.  whilst 
for  other  bricks  a  temperature  of  over  1,000°  C.  is  needed,  and  a 
certain  amount  of  fusion  occurs  before  the  full  red  colour  is 
produced.  As  the  colour  of  burned  bricks  is  due  to  the 
chemical  decomposition  of  the  clay,  it  cannot  be  predicted 
from  the  appearance  of  the  raw  clay.  Thus,  a  yellow  clay 
will  be  red  when  burned,  a  grey  clay  may  burn  to  a  yellow, 
red  or  blue  colour,  and  so  on,  and  white  bricks  may  be  made 
from  clays  of  almost  any  colour  except  a  bright  red  one.  The 
spotty  appearance  of  some  firebricks  is  due  to  the  grains  of 
pyrites  in  the  clay,  and  instead  of  being  regarded  as  a  sign  of 
inferiority  it  is  rather  an  indication  that  the  bricks  have 
proved  their  refractoriness  by  the  temperature  to  which  they 
have  been  heated  during  the  course  of  their  manufacture. 

The  best  qualities  of  firebricks  are  heated  to  temperatures 
higher  than  those  used  in  burning  any  other  kind  of  clay 


FINAL  STAGE  IN  BURNING  BRICKS  375 

goods,  but  they  are  so  resistant  to  heat  that  complete  vitrifica- 
tion is  never  reached.  It  is  essential  that  firebricks  should 
not  shrink  appreciably  when  in  use,  particularly  when  they 
are  employed  in  furnace  linings  ;  hence,  they  should  be  heated 
in  the  kiln  to  a  higher  temperature  than  they  are  ever  likely 
to  reach  afterwards.  Many  manufacturers  do  not  heat  their 
firebricks  to  temperatures  above  1310°  C.,  and  many  users 
consider  that  the  spaces  left  by  the  shrinkage  of  the  bricks 
in  use  are  unavoidable.  This  is  not  the  case,  as  if  the  bricks 
are  burned  at  a  sufficiently  high  temperature — which  obviously 
must  depend  on  that  of  the  furnace  in  which  they  will  be  used  — 
the  shrinkage  is  so  small  as  to  be  immeasurable.  For  annealing, 
brass  melting  and  much  other  work,  the  temperature  reached 
in  the  kilns  will  be  sufficient  to  prevent  the  bricks  shrinking 
still  further  in  use,  but  for  steel  manufacture,  regenerators, 
coke  ovens,  etc.,  harder-fired  bricks  must  be  employed.  In 
no  case,  however,  can  firebricks  be  said  to  be  "  fully  burned  " 
in  the  same  sense  in  which  this  phrase  is  applied  to  engineering 
and  other  well- vitrified  bricks.  The  resistance  to  heat  of  most 
firebricks  is  so  great  that  the  amount  of  vitrification  produced 
is,  necessarily,  very  small.  Where  bricks  of  only  a  moderate 
refractoriness  are  required  in  combination  with  a  high  degree 
of  resistance  to  abrasion  or  to  crushing,  it  is  often  possible  to 
use  vitrified  bricks  made  of  the  lower-grade  fireclays  or  of 
clays  which  vitrify  readily,  but  still  retain  their  shape  at  high 
temperatures. 

Firebricks  made  of  silica  instead  of  clay  are  described  in 
a  later  chapter.  They  are  characterised  by  their  expanding 
when  heated  instead  of  contracting  like  bricks  made  of  clay. 


CHAPTER  XIV. 

THE    PROPERTIES    OF    BRICKS 

THE  properties  of  bricks  are  chiefly  of  a  physical  character, 
their  chemical  characteristics  being  confined  to  their  resistance 
to  acids  and  to  the  action  of  water  and  other  atmospheric 
conditions. 

The  chemical  composition  of  bricks  is  seldom  of  any 
significance,  though  some  architects  and  engineers  attach  to 
it  a  wholly  unwarranted  importance.  As  previously  stated, 
the  analysis  of  a  clay  is  only  of  value  for  investigational 
purposes,  and  even  then  it  is  of  much  less  importance  than  other 
tests.  The  same  remark  is  equally  applicable  to  bricks. 

The  one  possible  exception  where  an  analysis  may  prove 
useful  is  in  the  case  of  firebricks,  as  a  clay  with  more  than 
2  per  cent,  of  lime,  magnesia  and  alkalies  can  scarcely  be 
expected  to  be  highly  refractory.  Even  in  this  case,  however, 
a  direct  determination  of  the  refractoriness  is  of  greater  value 
than  an  analysis. 

Bricks  are  commonly  classified  according  to  their  texture 
and  colour,  though  the  mode  of  manufacture,  hardness  and 
some  other  properties  must  also  be  taken  into  consideration, 
as  will  be  observed  in  the  following  definitions. 

Cutters  and  rubbers  are  the  softest  bricks  used  for  building 
purposes.  They  have  a  uniform,  sandy  texture,  and  are  easily 
cut  or  rubbed  to  any  desired  shape.  Their  chief  use  is  for 
arches  and  other  gauged  work.  The  best  bricks  of  this  class 
are  made  of  sandy  clay,  lightly  burned  in  clamps. 

Malms  are  yellow  bricks,  and  are  regarded  in  the  neighbour- 
hood of  London  as  one  of  the  best  kinds  of  building  bricks. 
They  are  composed  of  clay  and  chalk  to  which  cinder  dust 
has  been  added  and  are  burned  in  clamps,  with  the  result  that 
they  have  an  irregular  colour  and  texture,  with  a  considerable 
amount  of  vitrified  matter. 


VARIOUS    BRICKS  377 

Stocks  are  common  building  bricks,  and  vary  greatly  in 
texture  and  other  properties,  according  to  the  materials  used 
and  the  method  of  manufacture.  They  are  bricks  left  when 
the  most  regularly  coloured  ones  have  been  sorted  out,  and 
are  used  for  inside  work,  cellars,  etc.,  but  not  for  facing  work. 
In  some  localities,  however,  the  whole  of  the  contents  of  a 
kiln  are  sold  as  "  stocks,"  the  builders  selecting  such  facing 
bricks  as  they  require  after  the  bricks  have  been  delivered  to 
the  site.  Rough  stocks,  as  their  name  implies,  are  somewhat 
irregular  in  colour,  surface  and '  shape,  and  are  an  inferior 
kind  of  stock  brick  used  (on  account  of  cheapness)  for  founda- 
tions and  other  work  where  appearance  is  of  small  importance. 
Grey  stocks  are  so  termed  because  of  their  colour  ;  apart  from 
this,  they  are  good  bricks.  Grizzles  are  underburned  bricks  ; 
they  have  a  loose,  porous  texture,  and  are  soft  and  not  very 
durable  when  exposed  to  the  weather.  Crozzles,  burrs  and 
clinkers  are  bricks  which  have  partly  lost  their  shape  through 
overheating.  They  have  a  vitrified  texture  and  are  heavy, 
dense,  and  "  ring  "  when  struck.  The  term  clinker  is  also 
used  for  well- vitrified  paving  bricks  of  a  good  quality.  This 
double  use  of  the  term  is  liable  to  prove  confusing.  Bats  or 
brickbats  are  broken  bricks  or  those  which  are  so  misshapen 
as  to  be  quite  useless.  They  are  the  "  unsaleable  residue  "  of 
the  brickyard.  Shuffs,  shivers  and  shakes  are  bricks  which, 
in  course  of  manufacture,  have  become  cracked  either  internally 
or  externally,  and  are  consequently  unsound.  No  matter  how 
uniform  their  texture  apart  from  the  cracks,  or  how  good  their 
colour,  they  are  practically  useless  except  for  low  walls  and 
other  work  where  little  or  no  strength  is  needed. 

Engineering  bricks  and  paving  bricks  have  a  close,  well- 
vitrified  texture,  and  are  noted  for  their  durability,  impervious- 
ness  and  strength.  On  account  of  their  clinking  or  ringing 
note  when  struck,  these  bricks  are  sometimes  termed  clinkers. 

In  some  localities  the  bricks  which  have  been  laid  flat  in 
a  clamp — forming  the  "paving" — are  termed  paviours. 
They  are  not  appreciably  different  from  the  stocks  with  which 
they  are  associated,  and  must  not  be  confused  with  pavers, 
which  are  bricks  used  in  the  construction  of  floors  anc\ 
pavements. 


378  THE  PROPERTIES    OF    BRICKS 

The  shapes  and  sizes  of  bricks  are  exceedingly  varied, 
according  to  the  custom  of  the  locality  and  the  purposes  for 
which  they  are.  used.  Thus,  the  standard  brick,  as  denned 
by  the  Royal  Institute  of  British  Architects  in  1904,  measures 
—length  between  8|  and  9  inches  ;  breadth,  between  4  ^ 
and  4 §  inches  ;  thickness,  between  2|  and  2j J  inches,  but 
bricks  of  larger  and  smaller  sizes  are  made  in  many  works. 
Thinner  bricks  (some  of  them  only  one  inch  in  thickness)  are 
also  used  in  large  quantities,  as  are  bricks  with  one  corner  cut 
off  or  ornamented  (squints),  bricks  with  one  rounded  corner 
(bullnoses),  bricks  with  an  ornamental  end  (jambs),  or  with  an 
ornamental  face  (diamond  stretcher,  dog-tooth  stretcher,  half- 
moon  stretcher,  string  course  bricks,  etc.).  Some  bricks  are  also 
made  of  special  shapes  for  arches,  channels,  copings,  etc.  ; 
others  are  perforated  to  make  them  lighter  or  in  order  that 
they  may  serve  as  ventilators,  etc.  The  surface  is  sometimes 
scored  to  prevent  slipping,  as  in  paving  and  stable  bricks,  or 
a  depression,  or  frog,  is  made  in  one  or  both  sides  of  the  brick, 
partly  in  order  that  the  mortar  may  grip  more  firmly  and  partly 
to  reduce  the  weight  of  the  brick  without  affecting  its  size. 

Ornamental  bricks  are  made  in  an  endless  variety  of  shapes 
and  sizes,  the  modelled  work  on  some  being  exceedingly 
elaborate.  It  is  usually  better,  however,  to  have  the  more 
highly  decorative  work  on  larger  pieces  (terra-cotta),  as  the 
joints  of  ordinary  brickwork  are  a  disfigurement  to  modelled 
or  carved  work.  The  modelling  should  be  completed  before 
the  material  is  burned,  as  if  sculptured  afterwards  the  outer 
"  skin  "  which  forms  the  most  durable  surface  of  the  material  is 
destroyed  by  the  carving. 

Irregularity  in  the  size  of  bricks  is  a  source  of  great  annoyance 
to  bricklayers,  and  causes  unsightly  work.  It  is  sometimes 
due  to  the  use  of  inaccurate  moulds  or  to  allowing  one  or  more 
moulds  or  presses  to  become  unduly  worn,  whilst  others  in 
use  at  the  same  time  are  of  the  correct  size.  It  is  occasionally 
due  to  the  different  temperatures  reached  in  different  parts  of 
a  kiln  or  to  variations  in  the  temperature  of  the  same  kiln  on 
different  occasions.  Some  irregular  bricks  are  produced  in 
almost  every  kiln,  but  if  the  proportion  is  large  it  indicates 
carelessness  or  ignorance  in  manufacture  ;  they  are  removed 


THE    COLOUR   OF  BRICKS  379 

from  the  better  qualities  by  sorting  the  bricks  as  they  come 
from  the  kiln. 

The  surface  of  bricks  is  of  moderate  smoothness  ;  that  of 
terra-cotta  is  smoother,  and  glazed  bricks  have  a  surface 
resembling  that  of  glass.  Too  smooth  a  surface  is  considered 
undesirable  for  building  bricks,  as  a  moderate  degree  of  rough- 
ness increases  the  adhesion  of  the  mortar  and  enables  the  bricks 
to  present  a  more  pleasing  appearance  than  would  otherwise 
be  the  case.  For  the  last-named  reason,  some  brickmakers 
have  for  many  years  past  covered  their  bricks  with  red- 
burning  sand  (sand-faced  bricks,  p.  334),  whilst  others  use  some 
form  of  vibrating  wire  to  produce  a  special  form  of  roughness 
of  face. 

In  glazed  bricks,  the  surface  should  be  as  glossy  as  possible, 
free  from  pinholes  and  cracks,  and,  if  the  bricks  are  to  be 
exposed  to  trying  conditions,  the  glaze  should  be  impermeable 
to  red  ink. 

The  colour  of  bricks  varies  with  the  purposes  for  which  they 
are  required.  As  stated  on  a  previous  page,  the  majority  of 
brick  manufacturers  endeavour  to  obtain  bricks  of  as  uniform 
a  colour  as  possible,  though  some  architects  prefer  a  play  of 
colours  produced  by  irregularities  in  the  composition  of  the 
material  and  in  the  stoking  of  the  fires.  A  perfectly  uniform 
colour  can  only  be  obtained  with  materials  of  great  fineness 
which  are  extremely  well  mixed.  Coarse  particles,  imperfect 
mixing  and  materials  of  a  different  composition  to  the  main 
mass  tend  to  produce  irregular  colouring. 

A  whitish  scum,  or  efflorescence,  on  the  surface  of  bricks  may 
be  produced  by  soluble  salts  in  the  clay  or  by  condensation 
products  which  are  deposited  whilst  the  bricks  are  in  the  kiln. 
These  salts  dissolve  in  the  water  used  to  make  the  clay  plastic, 
the  solution  is  gradually  drawn  to  the  surface  as  the  bricks 
dry,  and  as  the  water  evaporates  the  salts  are  deposited  on 
the  surface  of  the  bricks. 

Some  bricks  have  a  white,  or  grey  face  or  exterior,  but  are 
red  internally.  The  white,  or  grey,  coating  is  due  to  the 
kiln  gases  condensing  on  the  bricks.  The  moisture  in  these 
gases  is  condensed  into  minute  drops  of  water,  and  any  sulphuric 
or  other  acids  present  are  dissolved  by  these  drops.  The 


380  THE  PROPERTIES   OF   BRICKS 

dilute  acid  solution,  so  formed,  dissolves  some  of  the  bases  in 
the  clay  and  forms  a  scum  similar  to  that  described  in  the 
foregoing  paragraph,  but  usually  distributed  more  uniformly 
over  the  surface  of  the  bricks.  Bricks  of  this  character  do 
not  receive  any  more  heating  than  red  bricks,  as  the  grey  or 
white  coating  is  produced  at  temperatures  far  below  red  heat. 
Notwithstanding  this  fact,  many  architects  consider  these 
bricks  to  be  better  and  stronger  than  if  the  same  material 
was  burned  so  as  to  produce  bricks  of  a  uniform  red  colour. 
The  origin  of  this  erroneous  opinion  is  obscure,  but  is  apparently 
due  to  confusion  of  scummed  red  bricks  with  grey  engineering 
bricks  of  an  entirely  different  character. 

When  the  natural  colour  of  bricks  is  too  irregular  and  dis- 
pleasing for  facing  purposes,  the  bricks  are  sometimes  coated 
with  a  special  engobe  made  of  a  red-burning  clay  of  superior 
colour.  These  coated  bricks  are  naturally  regarded  as  inferior, 
as  the  coating  tends  to  wear  and  chip  off  leaving  the  discoloured 
interior  visible.  In  a  few  cases,  bricks  are  dipped  in  a  special 
slurry  after  they  have  been  withdrawn  from  the  kiln,  in  order 
to  give  them  a  red  surface.  Such  bricks  may  be  compared 
to  bricks  which  are  merely  painted  to  hide  their  defects. 

The  hardness  of  bricks  varies  greatly.  Most  building  bricks 
are  rather  harder  than  sandstone,  and  the  best  vitrified  paving 
and  engineering  bricks  are  too  hard  to  be  scratched  by  an 
ordinary  steel  knife. 

Soft  bricks  are  regarded  as  having  been  insufficiently  heated, 
and  are  termed  "  underburned."  They  occur  in  those  parts 
of  the  kiln  where  the  average  temperature  is  not  reached,  and 
are  particularly  numerous  in  kilns  which  are  badly  designed 
or  which  have  been  improperly  fired.  Sometimes  bricks  which 
are  seriously  underfired  may  be  returned  to  the  kiln  and  burned 
a  second  time,  but  this  obviously  causes  a  serious  increase  in 
the  cost  of  manufacture. 

Soft,  friable  bricks  are  weak  and  therefore  of  more  limited 
use  than  stronger  bricks  in  which  the  particles  are  more  firmly 
cemented  together  by  the  fused  material  present. 

The  resistance  to  abrasion  shown  by  the  more  vitrified  bricks 
is  very  considerable,  and  is  an  important  property  where 
bricks  are  used  for  pavements,  roadways,  etc.  In  the  United 


THE    DENSITY    OF   BRICKS  381 

States,  paving  bricks  are  tested  by  placing  them  in  a  barrel 
or  tumbler  with  heavy  pieces  of  iron  and  rotating  for  a  definite 
time,  usually  1,000  revolutions  at  the  rate  of  thirty  per  minute. 
The  loss  in  weight  undergone  by  the  bricks  is  regarded  as 
indicating  their  lack  of  durability.  In  Germany  the  brick  is 
held  by  a  weight  on  to  the  surface  of  a  grinding  table  and  the 
amount  worn  away  is  ascertained.  Neither  of  these  tests  is 
really  accurate,  though  useful  as  a  rough  method  of  sorting 
out  the  worthless  bricks.  Resistance  to  wear  and  tear  is, 
largely,  dependent  on  the  toughness  of  the  bricks,  and  no 
means  have  yet  been  devised  for  accurately  measuring  this 
property.  It  appears,  however,  that  the  more  fused  material 
a  brick  contains,  i.e.,  the  more  completely  it  is  vitrified  the 
greater  will  be  its  toughness,  though  some  very  completely 
vitrified  bricks  are  too  brittle  to  be  of  value.  Bricks  made  of 
glass  are  quite  useless  for  paving  roads  because  of  their 
brittleness. 

Brittleness  is  an  undesirable  property  in  bricks,  and  is  most 
noticeable  in  those  which  have  been  heated  excessively,  though 
in  some  of  the  softest  and  underburned  bricks  the  particles  have 
so  little  power  of  cohesion  that  the  bricks  appear  to  be  brittle. 
Bricks  which  have  been  properly  burned,  but  cooled  too 
rapidly,  are  usually  brittle  because  of  the  strains  set  up  by 
the  rapid  cooling. 

The  density  of  bricks  is  the  ratio  of  their  weight  to  that  of  an 
equal  volume  of  water,  but  the  term  is  sometimes  used  in  a 
different  sense  with  reference  to  their  texture.  The  porous 
nature  of  most  bricks  makes  it  difficult  to  ascertain  their 
density  with  accuracy,  and  it  is  better  to  use  the  term  specific 
gravity  in  reference  to  the  true  density  of  the  material  itself, 
apart  from  the  pores  it  may  contain.  The  apparent 
density  may  then  be  understood  to  relate  to  the  weight  of  a 
brick  including  its  pores,  relative  to  that  of  an  equal  volume 
of  water.  The  practical  value  of  a  study  of  changes  in  the 
specific  gravity  and  apparent  density  of  bricks  during  various 
stages  of  burning  is  important,  as  it  enables  the  degree  of 
vitrification  at  different  temperatures  to  be  compared,  and, 
apart  from  any  consideration  of  the  colour  of  the  finished 
brick,  shows  the  temperature  at  which  the  heating  of  the  kiln 


382  THE   PROPERTIES    OF   BRICKS 

should  stop  and  the  nature  of  the  heating  which  is  most 
suitable  for  any  particular  material.  These  tests  are,  however, 
best  relegated  to  an  expert  in  clay  testing,  or  erroneous  con- 
clusions may  be  reached.  Apart  from  this  and  from  specula- 
tions on  the  precise  chemical  constitution  of  the  substances 
which  exist  in  burned  clays,  the  determination  of  specific 
gravity  and  apparent  density  are  of  minor  importance. 

Heavy  bricks  are  usually,  but  not  necessarily,  strong  ones, 
and  are  of  a  more  vitrified  nature  than  the  lighter  porous 
bricks. 

Light  bricks  are  valuable  in  some  localities  because  of  the 
lower  cost  of  carriage  per  thousand  bricks.  The  cause  of  their 
low  apparent  density  is  their  great  porosity  ;  this  may  be  a 
natural  property  of  the  clay,  but  it  is  usually  due  to  the  addition 
of  sawdust  or  some  other  combustible  material  of  the  clay. 
This  combustible  matter  burns  away  in  the  kiln  and  leaves  a 
brick  of  normal  volume,  but  highly  porous,  and  so  of  very  small 
weight.  The  properties  of  these  bricks  are  such  that  they 
can  only  be  used  for  inside  work  where  great  strength  is  not 
required. 

The  texture  of  bricks  is  best  observed  by  breaking  them  and 
studying  the  fractured  surface.  It  will  then  be  seen  that  the 
texture  is  that  of  an  amorphous  material  of  a  somewhat 
spongy  or  porous  character,  the  particles  being  held  firmly 
together  by  a  glassy  substance  which  is  formed  by  the  action 
of  heat  on  the  more  fusible  portion  of  the  material.  Hence, 
the  texture  of  bricks  in  some  ways  resembles  that  of  building 
stones,  but  is  more  homogeneous  and  the  particles  of  material 
are  usually  smaller. 

Bricks  of  perfectly  uniform  texture  are  costly  to  produce 
from  some  materials,  and  when  shales  and  other  indurated 
clays  or  boulder  clays  are  used,  the  bricks  frequently  contain 
pieces  as  large  as  peas,  which  produce  an  irregular  texture ; 
this  is  often  an  indication  of  weakness. 

A  well-made  brick  should  present  as  uniform  an  appearance 
as  possible  when  it  is  broken,  and  the  fractured  faces  are 
examined  with  the  unaided  eye.  If  a  good  magnifying  lens  of 
the  Coddington  type,  or  a  microscope,  is  used,  the  irregularities 
in  the  material  will  be  made  more  apparent,  localised  patches 


THE   POROSITY   OF   BRICKS  383 

of  fused  material  being  found  irregularly  distributed  throughout 
the  brick.  In  a  low  grade  of  brick  the  presence  of  various 
materials  other  than  clay  may  usually  be  recognised  without 
much  difficulty,  though  the  definite  identification  of  them 
with  certain  minerals  is  not  always  possible. 

Though  not  often  found,  the  best  texture  of  a  brick  is  that 
which  consists  of  a  complex,  felted  mass  of  minute  crystals, 
due  to  maintaining  the  bricks  for  a  considerable  time  at  a 
temperature  somewhat  above  that  at  which  vitrification  is 
first  noticeable.  The  risk  of  spoiling  the  bricks  completely 
makes  the  majority  of  manufacturers  unwilling  to  prolong  the 
heating,  and  consequently,  in  most  bricks,  this  crystalline, 
felted  texture  is  replaced  by  a  main  mass  of  amorphous 
particles  cemented  together  by  a  kind  of  glass. 

Apart  from  showing  any  great  irregularities  in  the  texture,  a 
microscopic  examination  is  of  little  value  to  the  engineer  and 
builder,  as  the  porosity  and  crushing  strength  yield  similar 
information  in  a  simple  form. 

The  porosity  of  bricks  is  an  important  characteristic  in  two 
respects  :  the  more  porous  a  brick  the  less  will  be  the  cost  of 
transport,  and  the  better  will  be  its  ventilating  power  ;  on 
the  other  hand,  the  more  vitrified  and  dense  a  brick  the  better 
will  it  be  able  to  resist  the  action  of  frost  and  rain.  It  is 
therefore  necessary  for  the  manufacturer  to  produce  bricks 
which  shall  not  be  so  porous  that  walls  made  of  them  remain 
damp,  or  bricks  which  are  so  impervious  that  no  air  can  pass 
through  them.  For  ordinary  building  bricks  a  porosity 
corresponding  to  the  absorption  of  water  equal  to  6  to  15  per 
cent,  of  the  weight  of  the  bricks  is  found  to  be  generally 
satisfactory,  but  for  bridges,  sewers  and  other  engineering 
work  bricks  which  are  completely  impervious  are  preferable. 

The  porosity  is  usually  determined  by  weighing  three  bricks, 
separately  immersing  each  in  water  in  such  a  manner  that  a 
piece  of  the  brick,  about  9  x  4J  x  J  inch,  is  above  the  water 
level.  After  half  an  hour  the  brick  is  immersed  completely 
and  is  left  for  twenty-four  hours.  At  the  end  of  this  time  each 
brick  is  taken  out,  rapidly  wiped  dry  with  a  smooth  cloth,  so 
as  to  remove  the  water  adherent  to  the  surface,  and  is  re- 
weighed.  The  increase  in  weight  is  due  to  the  water  absorbed 


384  THE   PROPERTIES    OF   BRICKS 

by  the  pores  in  the  brick,  and  is  calculated  to  a  percentage  on 
the  weight  of  the  original  brick.  Thus,  if  a  brick  weighs 
9J  Ibs.  ==  148  ounces  before  immersion,  and  163|  ounces 
afterwards,  its  porosity  will  be  163J  --  148  =  15  J  ounces  in 
148  ounces,  or  10  J  per  cent.  The  object  of  a  partial  immersion 
at  first  is  to  permit  the  air  to  be  displaced  from  the  pores  by 
the  water.  A  more  accurate  method  consists  in  breaking  the 
brick,  immersing  it  in  water  and  extracting  the  air  with  a 
vacuum  pump,  but  this  does  not  so  nearly  represent  the 
porosity  under  the  normal  conditions  of  use  as  the  form  of 
test  first  described,  as  the  protective  action  of  the  dense 
"  skin  "  on  the  surface  of  the  brick  is  nullified  by  breaking 
the  brick. 

The  porosity  of  samples  cut  from  the  interior  of  bricks  is 
often  useful  for  studying  the  extent  to  which  the  burning  has 
been  carried.  If  sufficient  care  be  taken  in  the  determina- 
tion, the  amount  of  porosity  will  indicate  the  amount  of 
fused  matter  which  has  been  formed  in  the  brick,  and  will, 
therefore,  show  the  conditions  under  which  bricks  of  the 
greatest  strength  can  be  made  from  the  material  under 
examination.  So  many  considerations  enter  into  this  question, 
however,  that  it  is  best  left  in  the  hands  of  a  competent  expert, 
as,  otherwise,  very  erroneous  conclusions  may  be  drawn. 

The  permeability  to  water  shown  by  some  bricks  is  important 
on  some  occasions.  For  ordinary  buildings,  bricks  should  be 
permeable  to  air,  but  less  so  to  water,  in  order  that  the  buildings 
may  "  breathe  "  through  the  brickwork  and  yet  not  have 
damp  walls.  The  best  bricks  for  this  purpose  are  those  which 
have  a  sufficient  porosity  to  retain  more  water  than  will 
impinge  on  them  by  the  heaviest  rainfall  to  which  they  are 
subject,  without  the  pores  being  so  large  that  the  inside  of  the 
wall  becomes  damp.  Walls  built  of  such  bricks  will  dry 
between  the  showers  and  will  preserve  a  dry  and  warm  interior 
to  the  building  of  which  they  form  a  part. 

The  strength  of  bricks  is  invariably  expressed  in  terms  of 
the  weight  required  to  crush  them.  Bricks  vary  greatly  in 
this  respect,  those  which  are  the  most  vitrified  (i.e.,  the  ones 
which  have  been  the  most  intensely  heated)  being  the  strongest. 
In  this  connection  it  is  important  to  remember  that  the  strength 


THE   STRENGTH   OF    BRICKS  385 

of  even  a  poor  brick  is  much  greater  than  that  of  a  well-built 
wall  made  of  good  bricks,  as  the  mortar  used  in  jointing  is 
not  as  strong  as  the  bricks.  The  demands  of  architects  that 
bricks  shall  show  a  minimum  crushing  strength  are,  therefore, 
somewhat  beside  the  mark,  and  are  chiefly  of  value  in  checking 
the  general  nature  of  the  bricks  specified  rather  than  in  showing 
their  actual  strength  when  in  use. 

The  strength  of  bricks  is  affected  by  the  precise  chemical 
and  physical  nature  of  the  raw  materials,  the  amount  of  water 
added,  the  general  treatment  in  mixing,  shaping,  and  burning, 
as  well  as  by  the  manner  of  cooling.  Hence,  no  two  bricks 
have  precisely  the  same  crushing  strength.  Figures  which  state 
that  "  Staffordshire  blue  bricks  "  have  a  crushing  strength  of 
800  tons  per  square  foot  are,  therefore,  misleading  to  the 
extent  that  some  blue  bricks  made  in  Staffordshire  are  much 
weaker  (300  tons  per  square  foot)  and  others  are  much  stronger 
than  the  figure  mentioned.  Even  in  the  same  works  the 
strength  of  the  bricks  varies  greatly  at  different  times,  and 
to  assume  that  because  certain  bricks  made  at  one  time  have 
a  satisfactory  crushing  strength,  therefore  all  the  bricks  made 
by  the  firm  will  be  equally  strong,  is  entirely  to  misunderstand 
the  facts. 

Comparison  of  the  crushing  strengths  of  bricks  of  different 
characters  and  from  different  parts  of  the  country  is,  therefore, 
of  little  value.  The  following  figures,  obtained  from  a  very 
large  number  of  samples  by  the  author,  in  the  course  of  his 
practice  as  a  consulting  expert  in  brick  manufacture,  etc., 
represent  about  as  fair  an  average  as  the  widely  divergent  results 
obtained  from  different  bricks  of  the  same  class  permits  : — 

Tons  per 
Crushing  strength.  square  foot. 

London  grey  stock  bricks         .          ,  92 

Suffolk  white  bricks  (Gault)     ,          .  139 

Essex  red  sand  stocks     ....          94 

Leicester  red  bricks  (wire-cut)  .  .        273 

South  Yorkshire  bricks  (stiff-plastic)  .        543 
Fletton  bricks  (semi-dry  process)      .  253 

Staffordshire  blue  bricks          .          .  .       790 

Rubbers  and  cutters  (very  variable)  .         70 
C.  C  C 


386  THE   PROPERTIES   OF    BRICKS 

In  Germany  no  building  bricks  may  be  used  which  have  a 
crushing  strength  below  136  tons  per  square  foot. 

The  principal  assurance  required  by  the  architect  and 
engineer  is  not  the  total  strength  of  the  bricks  when  new,  but 
their  strength  after  they  have  been  in  use  for  some  time. 

The  durability  of  bricks  is  of  greater  importance  than  their 
strength,  for  if  a  brick  decays  rapidly,  its  primary  strength  is 
of  no  importance.  Broadly  speaking,  the  durability  of  a 
brick  will  be  greatest  when  the  porosity  is  least,  but  as  com- 
pletely impervious  bricks  are  undesirable  for  many  structures, 
too  much  stress  must  not  be  laid  on  a  low  percentage  of  porosity. 
Again,  some  bricks,  notwithstanding  their  impermeability,  are 
so  weak  as  to  be  far  from  durable  under  a  heavy  load,  and 
others  are  so  affected  by  vibrations  as  to  be  useless  where 
heavy  machinery  is  employed.  The  durability  of  bricks 
depends  primarily  on  the  closeness  of  their  texture  and  on 
their  resistance  to  the  action  of  very  dilute  acids. 

Air  itself  has  no  power  to  destroy  bricks,  but  the  gases,  water 
and  salts  it  contains  can  do  so.  Thus,  the  small  proportion  of 
carbonic  acid  present  in  the  air  is  sufficient  to  attack  any  free 
lime  in  the  bricks  and,  eventually,  to  cause  it  to  "  blow," 
forming  unsightly  white  patches  and  cavities  on  the  surface, 
or  actually  cracking  the  bricks.  No  matter  how  fine  may  be 
the  particles  of  lime  in  the  brick,  if  the  latter  is  porous  they 
will  become  carbonated  in  the  course  of  time. 

The  action  of  rain  is  also  much  greater  on  a  porous  brick 
than  on  an  impervious  one.  The  water  has  a  solvent  action 
on  some  of  the  materials,  including  the  lime,  and  so  washes 
them  away  ;  at  the  same  time  it  has  a  softening  action  on 
imperfectly  burned  clay,  rendering  it  easier  for  subsequent 
rains  to  remove  such  material.  If  a  brick  absorbs  a  certain 
amount  of  rain  and  is  then  exposed  to  frost,  the  water  will 
expand  and  so  set  up  strains  which,  in  time,  will  cause  the 
brick  to  crumble  away.  Vegetable  matter,  such  as  lichens, 
mosses  and  algse,  also  effect  the  destruction  of  bricks  on  to 
which  they  settle  from  the  air,  the  rain  keeping  them  moist 
and  permitting  them  to  thrive  if  the  bricks  are  sufficiently 
porous  and  rough  of  surface.  Some  bricks  have  such  small 
pores  and  so  smooth  a  surface  that  the  lichens,  etc.,  cannot 


SCUM   IN   BRICKS  387 

adhere  to  them  ;  such  bricks  do  not  vegetate.  The  "  scum  " 
formed  by  vegetation  can  readily  be  recognised  under  the 
microscope. 

Bricks  behave  very  erratically  so  far  as  durability  is  con- 
cerned, and  cases  are  known  where  the  same  manufacturer  has 
supplied  bricks  for  two  houses  in  the  same  town  and  one 
house  has  suffered  far  more  than  the  other  from  the  weather. 

The  sheltered  or  exposed  nature  of  the  site  and  the  direction 
and  intensity  of  the  prevailing  winds  are  often  important 
factors  in  such  a  case.  Slight,  but  oft-repeated  changes  in 
temperature  are  another  cause  of  the  destruction  of  some 
bricks,  the  more  compact  and  vitrified  kinds  suffering  the  most 
damage.  In  very  porous  bricks  there  is  sufficient  space 
between  the  particles  to  absorb  the  expansion  due  to  heat  or  to 
allow  for  the  contraction  on  cooling,  but  impervious  bricks 
cannot  do  this.  The  damage  caused  by  changes  in  temperature 
is  most  serious  in  the  case  of  bricks  and  terra-cotta  which  have 
a  thin  dense  "  skin  "  on  the  surface  ;  this  "  skin  "  does  not 
expand  and  contract  at  the  same  rate  as  the  body  behind  it  and 
so  tends  to  peel  or  "  shell  "  off,  leaving  a  rough  under-surface 
which  rapidly  becomes  dirty  and  unsightly.  This  expansion, 
which  is  not  infrequently  permanent,  is  most  noticeable  in 
copings. 

A  cause  of  decay  which  is  not  usually  important  as  regards 
the  durability  of  the  bricks,  but  is  very  detrimental  to  their 
appearance,  is  the  formation  of  scum  or  efflorescence.  If  this 
does  not  occur  on  the  bricks  when  first  laid,  it  will  usually  be 
derived  from  the  mortar  or  from  the  foundation  on  which  the 
bricks  are  laid.  In  any  case  this  scum  or  efflorescence  (pro- 
viding that  it  is  not  of  a  vegetable  nature)  is  due  to  soluble 
salts  which  have  gained  access  to  the  bricks.  Usually  they 
have  been  dissolved  in  the  water  mixed  with  the  mortar  and 
have  dried  out  on  the  surface  of  the  bricks  as  the  mortar  dried, 
or  the  rain  falling  on  the  wall  has  dissolved  the  salts  out  of  the 
water  and  has  brought  them  to  the  surface  of  the  bricks  by 
capillary  attraction,  as  the  rain-water  evaporated.  An  alter- 
native and  by  no  means  infrequent  cause  of  scum  is  the 
presence,  in  the  foundations,  of  soluble  salts.  If  the  brickwork 
is  not  provided  with  an  adequate  damp-proof  course,  the 

c  c  2 


388  THE   PROPERTIES   OF   BRICKS 

ground  water  will  dissolve  these  salts  and  will  rise  up  in  the 
brickwork,  and  later,  as  it  dries  away,  it  will  leave  them  as  a 
scum  on  the  surface.  The  irregularity  of  the  patches  of  scum 
on  bricks  is  often  a  clear  sign  of  variations  in  their  porosity. 

Notwithstanding  all  that  has  been  stated  above  with  regard 
to  causes  which  lessen  the  durability  of  bricks,  the  fact  still 
remains  that  well-made  bricks  are  the  most  durable  building 
material  known.  Granite  and  all  other  stones  are  by  their 
very  nature  subject  to  decay  by  the  agency  of  the  weather, 
but  bricks  which  have  been  properly  made  and  burned  have 
been  found  to  outlast  the  hardest  stones.  The  remarkable 
state  of  preservation  of  many  ancient  Chaldean,  Assyrian, 
Egyptian,  Indian  and  other  buildings  is  due,  in  large  measure, 
to  the  fact  that  they  were  built  of  burned  clay.  The  climatic 
conditions  of  modern  industrial  towns  are  much  more  severe 
than  those  of  ancient  days,  owing  to  the  peculiarly  corrosive 
action  of  the  acids  contained  in  smoke  and  in  furnace  gases, 
yet  even  at  the  present  day  the  evidence  is  all  in  favour  of 
good  bricks  as  by  far  the  most  durable  building  material,  and, 
in  many  localities,  one  of  the  cheapest. 

Resistance  to  strong  acids  is  a  property  possessed  in  a  marked 
degree  by  many  vitreous  bricks  and  by  all  those  which  have 
been  glazed  by  the  aid  of  salt.  Porous  bricks,  if  free  from 
lime,  will  resist  the  action  of  strong  acids  for  a  time,  but  not 
for  long.  The  more  free  the  clay  is  from  iron,  lime  and 
magnesia  compounds  and  alkalies,  and  the  more  completely 
it  is  vitrified,  the  greater  will  be  its  resistance  to  acids. 

Bricks  which  are  rendered  impervious  by  a  coating  of  glaze 
will  only  resist  the  action  of  acids  so  long  as  the  glaze  is 
undamaged.  If  it  cracks,  spalls  or  is  broken,  the  resistance  of 
the  brick  will  be  very  small.  Hence,  the  use  of  glazed  bricks 
is  limited  to  those  cases  where  there  is  little  likelihood  of  the 
surface  being  damaged. 

The  refractoriness  or  resistance  to  very  high  temperatures 
possessed  by  firebricks  deserves  brief  mention  here.  Strictly 
speaking,  the  refractoriness  of  a  clay  is  its  power  of  resisting 
the  action  of  heat  alone,  quite  apart  from  any  accessory 
conditions,  but  the  term  is  generally  used  to  imply  resistance 
to  heat  when  used  in  furnaces,  boilers,  fireplaces,  etc.,  in  which 


THE  REFRACTORINESS   OF   BRICKS  389 

the  bricks  come  in  contact  with  coal  and  coke  ashes,  dust  of 
various  kinds,  slags  or  metal  drosses,  and  with  other  sub- 
stances, all  of  which  tend  to  reduce  their  resistance  to  heat. 
The  coal  in  contact  with  the  bricks  brings  about  reducing 
conditions,  and  effects  an  undue  slagging  of  the  iron  com- 
pounds ;  the  ashes,  slags,  dross,  dust,  etc.,  combine  with  the 
clay  and  form  a  more  fusible  material,  thereby  spoiling  the 
bricks  and  rendering  them  useless. 

Some  firebricks  are  very  sensitive  to  sudden  changes  in 
temperature,  and  crack  and  spall  (i.e.,  throw  off  portions  of 
material  from  the  surface)  if  cooled  suddenly,  as  when  a  furnace 
is  emptied,  or  the  fires  in  the  boiler  are  withdrawn.  This 
sensitiveness  is  not  due  to  lack  of  true  refractoriness,  but  to 
the  texture  of  the  material.  It  is  generally  found  that  highly 
porous  firebricks  are  capable  of  withstanding  the  most  rapid 
changes  in  temperature,  whilst  the  same  bricks,  when  heated 
more  strongly,  and  thereby  rendered  more  dense,  are  found  to 
spall  when  cooled  suddenly. 

For  lining  furnaces  used  in  metallurgical  operations,  fire- 
bricks must  be  highly  resistant  to  the  metal  and  the  corrosive 
slags  and  dross,  and  at  the  same  time  the  bricks  must  not  spall 
or  crack  with  the  sudden  drop  in  temperature  when  the  furnace 
is  emptied.  Such  firebricks  are,  therefore,  required  to  possess 
two  diametrically  opposite  characteristics — for  resistance  to 
corrosion  they  must  be  dense,  and  for  resistance  to  sudden 
changes  in  temperature  they  must  be  as  porous  as  possible. 
A  compromise  is  usually  effected  by  making  the  bricks  porous 
with  a  dense  face  or  surface. 

As  shrinkage  during  use  would  be  serious,  many  furnace 
builders  use  firebricks  made  of  silica  instead  of  clay,  as  silica 
bricks  expand  on  heating  instead  of  shrinking.  They  are, 
however,  less  refractory  than  the  best  fireclay  bricks.  Their 
manufacture  is  described  on  p.  399. 

The  selection  of  firebricks  for  various  purposes  would  be 
greatly  facilitated  if  their  properties  were  better  known  to  the 
users.  At  present,  the  only  basis  of  specification,  and  that 
of  only  a  semi-official  character  and  by  no  means  widely 
adopted,  is  the  one  published  by  the  Institution  of  Gas 
Engineers. 


390  THE  PROPERTIES   OF  BRICKS 

This  provides  that  a  material  made  from  fireclay  shall  contain 
approximately  not  more  than  75  per  cent,  of  silica.  It  is  known, 
however,  that  there  are,  in  fe&rtain  areas,  fireclays  containing  as  much 
as  80  per  cent,  silica,  and  material  made  from  such  clays  shall  be 
considered  to  conform  to  this  specification  if  it  passes  the  tests  herein 
specified. 

Clause  1.  Refractoriness. — Two  grades  of  material  are  covered  by  the 
specification : 

(1)  Material    which    shows    no    sign  of  fusion  when  heated  to   a 
temperature  of  not  less  than  Seger  cone  30  (about  1,670°  C.). 

(2)  Material  which  shows  no  sign  of  fusion  when  heated  to  a  tem- 
perature of  not  less  than  Seger  cone  26  (about  1,580°  C.). 

The  test  shall  be  carried  out  in  an  oxidising  atmosphere,  the 
temperature  of  the  furnace  being  increased  at  the  rate  of  about  50°  C. 
per  five  minutes. 

The  "new  scale"  of  Seger  cones  is  to  be  used. 

The  material  is  to  be  chipped  to  the  form  and  size  of  a  Seger  cone 
and  tested  against  standard  Seger  cones  (small  size). 

A  preliminary  trial  is  first  made  with  a  piece  of  the  material  chipped 
into  the  approximate  form  of  a  cone.  This  should  be  cemented  on  to 
a  refractory  disc  or  slab  with  a  mixture  of  alumina  and  best  china  clay, 
together  with  Seger  xxmes  28,  30  and  32  (small  size).  It  is  essential, 
however,  that  the  temperature  should  be  maintained  constant ;  and 
if  it  is  necessary  to  remove  plugs,  etc.,  for  the  purpose  of  obtaining 
the  temperature,  great  care  should  be  taken  to  avoid  cooling  the  furnace 
by  such  means.  The  cones  should,  in  all  cases,  be  placed  relative  to 
the  sample  so  that  both  are  subjected  to  the  same  temperature.  These 
cones  are  selected  because  they  cover  the  range  of  first-class  clays. 
Best  china  clay  fuses  between  cones  35  and  36  ;  and  all  British  fireclays 
fall  below  this  point.  If  cones  28  and  30  fall,  the  furnace  should  be 
cooled,  and  the  material  under  investigation  examined.  If  it  exhibits 
no  sign  of  fusion,  the  trial  should  be  repeated  with  cones  31,  32  and  33. 
When  cone  32  squats,  the  piece  should  be  again  examined,  and  if  it 
shows  signs  of  fusion,  the  trial  should  be  repeated  with  cones  30,  31 
and  32.  By  this  method  of  approximation,  it  is  possible  to  decide 
whether  the  piece  vitrified  between  cones  30  and  31  or  between[cones 
31  and  32.  A  similar  method  should  be  adopted  when  testing  second- 
grade  material. 

It  may  be  noted  that  clays  and  related  materials  have  no  sharply 
defined  melting  point,  and  the  definition  of  refractoriness  here  adopted 
refers  to  the  temperature  at  which  the  angular  edges  of  the  material 
under  investigation  begin  to  lose  their  angularity  when  heated. 

Clause  2.  Chemical  Analysis. — A  complete  chemical  analysis  of  the 
materials  is  to  be  provided  when  required  by  the  engineer  (or  purchaser), 
for  his  personal  information  only. 

The  silica  should  be  determined  by  two  evaporations  with  an  inter- 
vening filtration ;  and  the  alumina,  lime  and  magnesia  by  two 
precipitations.  The  amount  of  titanic  oxide  should  be  indicated,  and 
not  included  with  alumina  and  iron.  The  potash  and  soda  should  be 
separately  determined. 

Clause  3.  Surfaces  and  Texture. — The  material  shall  be  evenly 
burnt  throughout,  and  the  texture  regular,  containing  no  holes  or 
flaws.  All  surfaces  shall  be  reasonably  true  arid  free  from  flaws  or 
winding. 

Clause  4.  Contraction  or  Expansion. — A  test  piece,  when  heated  to 


SPECIFICATION   FOR   FIREBRICKS  391 

a  temperature  of  Seger  cone  14  for  two  hours,  shall  not  show  more 
than  the  following  linear  contraction  or  expansion : — 

Per  cent. 

No.  1  grade     ....       1 
No.  2      „  .          .          .       1-25 

After  the  test  temperature  has  been  reached,  the  furnace  shall  be 
maintained  constant  at  that  temperature  throughout  the  testing  period. 

The  test  piece  shall  be  4|  inches  long  and  4^  inches  wide,  the  ends 
being  ground  flat,  and  the  contraction  measured  by  means  of  Vernier 
calipers  reading  to  O'l  mm. — a  suitable  mark  being  made  on  the  test 
piece,  so  that  the  calipers  may  be  placed  in  the  same  position  before 
and  after  firing. 

The  contraction  referred  to  is  linear,  and  is  equal  to — 

change  in  length  X  100 
original  length  of  test  piece* 

A  pyrometer  is  required  to  ensure  the  temperature  being  maintained 
within  the  proper  limits. 

Clause  5.  Variations  from  Specified  Measurements. — In  the  case  of 
ordinary  bricks,  9  inches  by  4|  inches,  by  3  inches  or  2-|  inches  thick, 
there  shall  not  be  more  than  +  li  per  cent,  variation  in  length,  nor 
more  than  +  per  cent,  variation  in  width  or  thickness,  and  in  all  cases 
the  bricks  shall  work  out  their  own  bond,  with  not  more  than  |-inch 
allowance  for  joint.  In  the  case  of  special  bricks,  blocks,  or  tiles, 
there  shall  not  be  more  than  +  2  per  cent,  variation  from  any  of  the 
specified  dimensions. 

Clause  6.  Crushing  Strength. — The  material  shall  be  capable  of 
withstanding  a  crushing  strain  of  not  less  than  l,8001bs.  per  square 
inch,1  when  applied  to  whole  bricks  placed  with  their  long  side  vertical 
between  the  jaws  of  the  machine,  giving  a  vertical  thrust.  The  most 
important  factors  requiring  attention  are : — 

(1)  The  two  ends  of  the  brick  which  come  in  contact  with  the  jaws 
of  the  machine  must  be  either  ground  or  sawn  flat  and  parallel,  so  as 
to  receive  a  vertical  thrust. 

(2)  An  average  of  not  less  than  three  bricks  must  be  used,  because 
flaws,  etc.,  may  give  an  abnormal  result,  which  might  not  be  detected 
if  only  one  brick  be  used. 

Clause  7.  Cementing  Clay  or  "  Fireclay  Mortar." — This  shall  be 
machine  ground,  and,  at  the  discretion  of  the  manufacturer,  may 
contain  a  suitable  percentage  of  fine  grog  ;  but  in  all  cases  the  cement 
clay  shall  be  quite  suitable  for  the  purpose  of  binding  together  the 
bricks,  blocks,  or  tiles  for  which  it  is  supplied,  and  shall  be  capable 
of  withstanding  the  same  test  for  refractoriness. 

Clause  8.  Marking  of  Material. — All  bricks,  blocks,  or  tiles  shall  be 
distinctly  marked  by  means  of  a  figure  1  or  2  (not  less  than  one  inch 
long)  stamped  on  them  to  indicate  the  grade  to  which  they  belong,  and 
it  shall  be  understood  that  any  material  not  so  marked  shall  be  ungraded, 
and  is  not  purchased  in  accordance  with  the  terms  of  this  specification. 

Clause  9.  Inspection  and  Testing. — The  engineer  (or  purchaser),  or 
his  agreed  representative,  shall  have  access  to  the  works  of  the  maker 
at  any  reasonable  time,  and  shall  be  at  liberty  to  inspect  the  manu- 
facture at  any  stage,  and  to  reject  any  material  which  does  not  conform 

1  This  figure  is  exceedingly  low,  and  is  only  85  per  cent,  of  the  minimum 
crushing  strength  permitted  for  common  building  bricks  in  Germany.  A.  B.  S. 


392  THE   PROPERTIES   OF   BRICKS 

to  the  terms  of  this  specification.  Pieces  may  be  selected  for  the 
purposes  of  testing,  either  before  or  after  delivery,  but,  in  either  case, 
a  representative  of  the  maker  shall,  if  he  choose,  be  present  when  such 
selection  is  made,  and  shall  be  supplied  with  a  similar  piece  of  the 
retort  material  to. that  taken  for  the  purpose  of  testing. 

If  the  engineer  (or  purchaser)  and  the  maker  are  not  prepared  to 
accept  each  other's  tests,  they  shall  agree  to  submit  the  samples  for 
testing  to  an  independent  authority  to  be  mutually  agreed  upon,  and 
the  engineer  (or  purchaser)  reserves  to  himself  the  right,  if  the  material 
does  not  conform  to  the  tests  laid  down  in  the  specification,  to  reject 
any  or  all  the  material  in  the  consignment  from  which  the  test  pieces 
were  taken. 

The  cost  of  these  independent  tests  and  of  any  retort  lengtLs  or 
tiles  damaged  before  delivery  for  obtaining  test  pieces,  shall  be  equally 
divided  between  the  purchaser  and  the  maker  if  the  test  proves  satis- 
factory, and  if  unsatisfactory  such  cost,  and  that  for  all  other  subsequent 
tests  required  on  this  account  from  the  same  consignment,  shall  be 
borne  by  the  makers. 

The  cost  of  any  tests  or  of  any  material  damaged  for  the  purpose  of 
obtaining  test  pieces  after  delivery  shall  be  borne  by  the  purchaser 
in  the  event  of  the  test  being  satisfactory,  and,  if  unsatisfactory,  by 
the  manufacturers,  in  a  similar  manner  to  that  specified  for  the  tests 
prior  to  delivery. 

It  is  suggested  that,  until  all  the  manufacturers  have  suitable  arrange- 
ments and  appliances  for  constantly  testing  their  goods,  it  may  be 
possible  to  render  them  some  assistance  by  allowing  a  fairly  large 
sample  of  their  material  to  be  sent  in  for  testing  and  general  approval 
before  extensive  deliveries  are  made.  This  is  in  no  way,  however,  to 
be  construed  as  removing  the  right  of  the  purchaser  to  test  material 
in  any  subsequent  consignment. 

Such  a  specification  cannot,  by  its  very  nature,  meet  the 
needs  of  more  than  a  few  users  of  firebricks,  for  the  following 
reasons : — 

(a)  The  specification  is  so  lenient  that  many  unsatisfactory 
firebricks  conform  to  it. 

(6)  It  does  not  recognise  the  relative  importance  of  various 
properties  under  given  conditions  of  use. 

(c)  It  has  been  based  upon  a  too  limited  knowledge  and 
experience  of  the  purposes  for  which  firebricks  are  used  and 
the  conditions  under  which  they  are  employed. 

In  selecting  firebricks  for  a  particular  purpose  it  is  important 
to  remember  that  the  actual  resistance  to  heat  alone  may  not 
be  the  most  important  factor.  It  has  already  been  stated 
that  the  strength  of  a  building  brick  depends  very  largely  on 
the  amount  of  vitrification  which  has  taken  place  in  it — i.e.,  on 
the  extent  to  which  the  fused  matter  has  bound  the  less  fusible 
particles  together.  The  same  is  equally  true  of  firebricks  so 


SPECIFICATION  FOR  FIREBRICKS  393 

far  as  the  load-carrying  power  of  the  cold  bricks  is  concerned. 
When  they  are  heated,  however,  the  glassy  material  in  them 
softens  and  is  a  source  of  weakness.  If,  therefore,  the  tempera- 
ture to  which  the  firebricks  will  be  exposed  in  use  is  not  very 
high — say  not  exceeding  1,100°  C. — it  is  preferable  to  use  a 
firebrick  which  shows  a  low  refractoriness,  corresponding  to 
cone  26  to  29,  in  preference  to  a  highly  refractory  one  (cone  34 
or  above).  Such  a  second-grade  or  third-grade  brick  will,  if 
it  has  been  properly  burned,  contain  more  fused  matter  than 
a  better  grade  of  brick,  and  will  be  better  able  to  resist  the 
action  of  various  abrasives  and  reducers  with  which  it  may  come 
into  contact,  and  will  be  better  able  to  withstand  rough  usage. 
If,  on  the  contrary,  the  temperatures  in  the  furnace  reach 
1,400°  C.  or  above,  the  choice  of  a  firebrick  is  much  more 
difficult.  It  is  then  necessary  to  ascertain  by  experiment 
what  importance  should  be  attached  to  strength  and  heat 
resistance  respectively,  it  being  always  borne  in  mind  that  as 
the  resistance  to  heat  is  increased  the  strength  is  diminished, 
and  vice  versa. 

For  this  reason,  it  is  usually  wise  to  build  different  portions 
of  the  furnace  with  different  kinds  of  firebricks.  Thus,  the 
dome  or  arch  will  require  to  be  built  of  bricks  whose  chief 
characteristic  is  strength  to  resist  the  crushing  tendency  due 
to  expansion  and  contraction,  combined  with  ability  to  with- 
stand sudden  changes  of  temperature  ;  in  these  bricks  resistance 
to  heat  per  se  is  of  secondary  importance.  The  walls  of  the 
furnace  will  require  to  be  more  heat-resisting,  whilst  still 
retaining  as  much  mechanical  strength  as  possible.  In  the 
hearth  or  hottest  part  of  the  furnace,  where  the  greatest  possible 
resistance  to  heat  is  required,  some  strength  must  be  sacrificed 
to  secure  refractoriness. 

It  is  futile,  if  the  best  results  are  desired  at  very  high  tempera- 
tures, to  expect  to  have  the  furnace  built  throughout  of  the 
same  materials,  and  for  the  same  reason  it  is  equally  futile 
to  expect  any  official  or  semi-official  specification  to  be  of  value 
except  in  'excluding  materials  which  have  so  low  a  resistance 
to  heat  as  to  be  unworthy  of  the  adjective  "  refractory."  To 
secure  the  maximum  of  durability  each  portion  of  a  furnace 
requires  a  different  specification,  and  even  then,  unless  the 


394  THE   PROPERTIES   OF   BRICKS 

type  of  furnace  and  its  uses  are  fully  recognised,  the  specification 
will  be  of  small  value. 

It  is  the  absence  of  any  proper  basis  of  specification  which 
is  the  cause  of  so  much  contention  and  dissatisfaction  between 
the  manufacturers  and  certain  users  of  firebricks.  In  many 
instances  neither  the  user  nor  the  manufacturer  knows  what 
properties  the  bricks  are  required  to  possess,  or,  if  a  selection 
of  properties  must  be  made,  which  are  the  most  important 
ones.  In  other  cases,  the  manufacturer  could  be  of  great 
assistance  if  only  the  user  would  describe  more  fully  the  precise 
purpose  for  which  the  bricks  are  required  and  for  which  part 
of  the  furnace.  In  the  most  difficult  cases,  the  best  results 
are  obtained  by  consulting  an  expert  who  is  independent  alike 
of  the  manufacturer  and  the  user,  and  whose  training  and 
experience  are  such  that  he  knows  what  neither  of  the  other 
parties  concerned  can  ascertain.  The  rapidly  increasing 
demand  for  furnaces  to  work  commercially  at  temperatures 
unattainable  ten  years  ago  has  made  the  consultation  of  such 
independent  experts  an  absolute  necessity  in  many  cases  and 
desirable  in  many  more. 

Other  properties  of  bricks  and  clays  which  have  been  burned 
in  kilns  will  be  found  described  in  the  author's  "  Modern 
Brickmaking,"  in  his  "  British  Clays,  Shales  and  Sands,"  and 
in  "  Bricks  and  Tiles,"  by  Dobson  and  Searle. 


CHAPTER  XV 

SILICEOUS     BRICKS 

BRICKS  made  of  clay,  or  of  earths  containing  a  large  propor- 
tion of  clay,  are  by  far  the  strongest  and  most  durable  as  a 
building  material,  but  in  localities  where  clay  is  scarce  and 
sand  is  plentiful,  the  latter  may  be  used.  Sand  possesses  so 
little  cohesion  that  the  grains  must  be  cemented  together  by 
the  aid  of  some  added  material.  The  nature  of  this  added 
material  determines  the  properties  of  the  bricks  made. 

Siliceous  bricks  are  of  three  main  kinds,  termed  respectively 
(a)  lime-sand  bricks,  (6)  cement-sand  bricks,  and  (c)  silica  fire- 
bricks. 

Lime-sand  bricks  are  made  by  mixing  sand  or  crushed 
sandstone  with  a  "  milk  of  lime."  This  material,  which 
consists  of  quicklime  suspended  in  water,  all  lumps  being 
removed  by  passing  it  through  a  fine  sieve  (No.  50),  is  mixed 
directly  with  the  sand  so  as  to  form  a  very  stiff  coherent  mass. 
Much  better  results  may  be  obtained  by  grinding  the  quicklime 
with  rather  more  than  an  equal  weight  of  sand  in  a  ball  mill 
until  the  mixture  is  fine  enough  to  pass  completely  through  a 
No.  50  sieve.  It  is  then  mixed  with  the  remainder  of  the 
(unground)  sand,  water  is  added  and  the  mixture  heated  so 
that  it  "boils"  whilst  passing  through  a  further  mixing 
machine.  The  proportion  of  lime  required  in  either  case  is 
between  6  and  10  per  cent,  of  the  weight  of  the  sand,  but  the 
proportion  which  gives  the  best  result  must  be  determined  by 
experiment  and  rigidly  maintained.  It  is  essential  that  no 
unslaked  lime  shall  be  present  in  the  mixture,  as  this  would 
make  the  bricks  unsound  and  weak.  Hence,  it  is  a  wise 
precaution  to  store  the  mixture  of  lime,  sand  and  water  in 
large  bins  or  silos  for  twenty -four  to  seventy -two  hours, 
in  order  that  the  water  may  be  uniformly  distributed  and  the 


396 


SILICEOUS  BRICKS 


LIME-SAND   BRICKS  397 

lime  completely  slaked.  The  stored  mixture  is  formed  into 
bricks  in  machines  capable  of  exerting  a  pressure  of  100  to 
150  tons,  and  similar  to  those  used  for  making  "  dry  "  or 
"  dust  "  bricks  (p.  343).  The  bricks  so  produced  are  then 
placed  on  waggons  and  heated  with  steam  at  about  125  Ibs. 
per  square  inch  pressure  for  eight  to  ten  hours  in  an  autoclave 
or  hardening  chamber.  On  removal  from  this  chamber  they 
are  ready  for  use,  but  their  strength  is  increased  and  their 
quality  is  improved  by  storing  them  in  open  sheds  for  several 
weeks,  or  even  months,  before  they  are  used. 

There  are  many  details  which  require  skilled  attention  in  the 
manufacture  of  these  bricks,  and  it  is  by  no  means  so  easy  to 
make  them  remuneratively  as  may  appear  to  be  the  case  from 
this  description.  With  suitable  sand  and  strict  and  skilful 
management,  however,  the  production  of  lime-sand  bricks  is 
by  no  means  so  difficult  as  that  of  bricks  made  of  clay,  as  the 
very  difficult  process  of  burning  is  entirely  avoided. 

Lime-sand  bricks  are  usually  white,  or  as  nearly  so  as  the 
natural  colour  of  the  sand  used  in  their  manufacture  permits 
them  to  be.  They  should  have  a  crushing  strength  of  at 
least  128  tons  per  square  foot,  and  are,  therefore,  quite  as 
strong  as  the  average  building  bricks  used  in  the  south  of 
England.  They  are  seldom  so  strong  as  the  best  bricks  made 
by  the  stiff -plastic  process  in  the  Northern  Midlands. 

As  "  sand  "  is  a  term  used  to  indicate  the  physical  nature  and 
not  the  composition  of  materials  to  which  it  is  applied,  the 
term  "  lime-sand  bricks  "  is  frequently  used  for  bricks  made 
from  ground  slag,  boiler  and  destructor  refuse  or  similar 
siliceous  materials.  These  clinker  bricks  are  made  in  the 
same  manner  as  bricks  made  from  sand  and  lime,  but  the 
chemical  reaction  which  occurs  when  the  crushed  slag  or  ashes 
are  mixed  with  water  renders  a  thorough  heating,  mixing  and 
slaking  even  more  essential  than  when  a  purely  siliceous  sand 
is  used.  Clinker  bricks  and  slabs  form  a  useful  means  of 
converting  an  inconvenient  waste  product  into  a  commercially 
valuable  one,  and  they  are,  therefore,  being  made  in  increas- 
ingly large  quantities  by  corporations  and  firms  with  sufficiently 
large  supplies  of  clinker  to  make  their  utilisation  possible  as 
a  commercially  profitable  process. 


398 


SILICEOUS   BRICKS 


Cement-sand  bricks  are  those  in  which  the  particles  of  sand 
or  other  crushed  siliceous  material  are  bound  together  with 


a, 
65 


a 


Portland  and  other  suitable  cement.  The  sand  is  screened  or 
washed  so  as  to  remove  gravel  and  other  coarse  particles,  and 
is  then  mixed  with  about  one-third  of  its  weight  of  Portland 


SILICA   BRICKS  399 

cement  and  with  sufficient  water  to  produce  a  suitable  paste. 
The  mixture — which  is  really  a  concrete  (see  p.  250) — is  then 
mixed  by  hand  or  in  a  mixing  machine,  and  is  filled  into  moulds 
and  tamped  until  a  film  of  water  rises  to  and  covers  the  surface. 
The  mould  is  then  removed  and  the  brick  allowed  to  stand  for 
a  few  weeks  until  it  is  fully  matured.  Cement-sand  bricks 
are  sometimes  made  in  power -presses  (Fig.  112),  but  they  are 
seldom  so  satisfactory  as  those  made  in  hand  presses  (Fig.  88) ; 
excessive  pressure  appears  to  disturb  the  arrangement  of  the 
particles  during  hardening.  As  the  process  of  moulding  is 
somewhat  slow,  it  is  customary  either  to  mould  six  or  more 
bricks  at  a  time  or  to  form  larger  blocks,  about  1J  cubic  feet 
each  (Fig.  86).  These  may  be  made  with  a  face  cut  to  imitate 
worked  stone,  by  the  insertion  of  a  suitable  die  in  the  mould. 

The  "  sand  "  used  in  cement-sand  bricks  need  not  consist 
of  a  siliceous  sand  ;  boiler  and  destructor  refuse,  ground 
furnace  slag  and  various  other  waste  products  may  be  used, 
providing  that  they  do  not  contain  raw  clay.  If  more  than 
about  3  per  cent,  of  raw  clay  is  present  the  bricks  or  blocks  will 
be  too  weak. 

For  a  full  description  of  the  processes  and  reactions  which 
occur  in  the  manufacture  of  cement-sand  bricks,  the  reader 
should  consult  the  section  on  concrete,  pp.  248,  et  seq.  The 
use  of  these  bricks  is  distinctly  limited  in  scope,  as  the  erection 
of  a  monolithic  concrete  structure  is  cheaper — except  in  the 
case  of  small  repairs — because-  in  addition  to  the  cost  of  making 
the  bricks  or  blocks  there  is  the  expense  of  laying  them.  The 
chief  use  of  such  bricks  or  blocks  is  thus  confined  to  localities 
or  circumstances  where  concrete  monolithic  work  is  undesirable 
or  impracticable. 

Silica  firebricks — sometimes  termed  "  Silica  bricks,"  Dinas 
bricks  or  Ganister  bricks — are  chiefly  used  in  metallurgical 
furnaces,  for  which  purpose  some  firms  prefer  them  to  bricks 
made  of  fireclay.  In  general  composition  they  resemble 
lime-sand  bricks  (p.  395),  but  contain  less  lime,  and,  in  order 
to  prevent  them  from  shrinking  when  in  use  in  the  hottest 
parts  of  the  furnace,  they  are  fired  in  kilns  previous  to  being 
sent  out  by  the  manufacturer. 

They  are  chiefly  made  from  a  pure  siliceous  rock  or  from  a 


400  SILICA    BRICKS 

similar  rock,  known  as  ganister,  which  contains  about  10  per 
cent,  of  clay,  and  occurs  in  the  Coal  Measures.  The  clay  in 
the  ganister  bricks  is  often  sufficient  to  provide  all  the  binding 
material  necessary  to  cement  the  particles  together,  but  the 
purer  siliceous  rocks  require  the  addition  of  about  2  per  cent, 
of  their  weight  of  quicklime.  The  proportion  of  lime  should 
be  kept  as  low  as  is  possibly  consistent  with  the  strength  of 
the  bricks,  as  it  reduces  their  refractoriness  (pp.  307,  388). 

The  crushed  rock  is  mixed  with  milk  of  lime  (p.  395)  and 
water  so  as  to  form  a  stiff  coherent  mass  similar  to  that  used 
in  the  manufacture  of  lime-sand  bricks.  The  paste  is  then 
taken  direct  from  the  mixer  to  hand  moulds,  where  it  is  formed 
into  bricks  under  a  slight  pressure.  The  bricks  are  then  dried 
in  a  warm  room  and  are  afterwards  taken  to  the  kiln,  where 
they  are  burned  under  such  conditions  that  cone  18  is  bent 
after  about  three  days.  Some  firms  are  content  to  bend 
cone  14,  but  the  additional  heating  secures  several  advantages 
and  greatly  increases  the  durability  of  the  bricks. 

Such  bricks,  when  broken,  have  a  texture  resembling  sand- 
stone, the  lime  having  combined  with  some  of  the  silica  to 
form  a  fusible  compound,  which,  on  cooling,  cements  the  grains 
of  silica  together.  Though  not  so  refractory  as  the  best 
fireclay  bricks,  silica  bricks  are  superior  to  low-grade  firebricks 
so  far  as  mere  resistance  to  heat  is  concerned.  They  are, 
however,  extremely  sensitive  to  sudden  changes  in  temperature, 
and  crack,  spall  and  peel  badly  when  cooled  suddenly.  Unlike 
fireclay  bricks,  they  expand  instead  of  shrinking  when  heated. 


CHAPTER  XVI 

BASIC   AND    NEUTRAL   BRICKS 

FOR  some  purposes  it  is  important  that  the  bricks  used 
should  have  a  definitely  basic  or  neutral  character.  This  is 
particularly  the  case  in  the  manufacture  of  certain  chemicals 
and  of  certain  metals  and  alloys. 

Basic  bricks  are  specially  useful  for  furnaces  in  which  slags 
rich  in  lime  are  produced,  as  they  are  unattacked  by  such 
slags,  whereas  acid  bricks  (made  of  fireclay  or  silica)  would 
rapidly  be  corroded,  owing  to  the  combination  of  the  acid  and 
the  base.  Basic  bricks  are  made  of  magnesia  or  lime,  but  the 
latter  are  so  weak  as  to  be  seldom  employed. 

Magnesite  bricks,  which  are  the  most  extensively  used  basic 
bricks,  are  made  by  cautiously  burning  magnesite  in  a  shaft 
kiln  similar  to  those  used  for  burning  lime.  The  magnesite 
is  thus  decomposed,  evolving  carbonic  acid  gas,  and  forming 
magnesia.  Some  of  the  material  is  drawn  from  the  kiln  after 
a  short  exposure  at  a  red  heat,  but  for  the  remainder  the 
heating  is  continued  until  the  magnesia  sinters  and  forms  a 
slaggy  mass.  The  caustic  magnesia  and  the  sintered  magnesia 
are  then  mixed  in  suitable  proportions  and  ground  to  a  fine 
powder  ;  a  little  water  is  added  and  mixed  thoroughly  with 
the  materials  so  as  to  form  a  stiff  paste.  The  paste  is  then 
compressed  in  powerful  hydraulic  presses  and  the  bricks  so 
produced  are  dried  and  then  burned  in  suitable  kilns.  The 
temperature  required  for  burning  magnesia  bricks  is  so  high 
(cone  18  to  23)  that  gas-fired  kilns  fitted  with  regenerators  are 
the  most  economical,  though  round  down-draught  kilns  are 
also  extensively  used  for  this  purpose. 

The  manufacture  of  magnesite  bricks  is  one  of  peculiar 
difficulty,  as  the  sintered  magnesia  is  difficult  to  grind,  the 
bricks  as  they  come  from  the  drying  chamber  are  exceedingly 
tender  and  must  be  handled  with  great  care,  and  the  high 
temperatures  in  the  kilns  are  by  no  means  easy  to  maintain. 

c.  D  D 


402  BASIC  AND  NEUTRAL   BRICKS 

The  production  of  these  bricks  is,   therefore,   confined  to  a 
limited  number  of  firms. 

Magnesite  bricks  are  particularly  sensitive  to  silica  and 
clay,  and  with  these  materials  form  a  glassy  slag  at  high 
temperatures.  It  is,  for  this  reason,  very  difficult  to  use  both 
magnesite  and  clay-  or  silica-bricks  in  the  same  structure. 

Bauxite  bricks  are  commonly  regarded  as  basic,  though  in 
many  ways  they  partake  of  a  neutral  character.  Bauxite  is 
the  mineral  name  for  a  variety  of  impure  alumina,  which 
usually  contains  considerable  proportions  of  iron  oxide  and 
combined  water.  The  bauxite  is  ground  to  powder,  mixed 
with  a  little  clay  and  water  to  form  a  stiff  paste,  and  s  then 
moulded  in  a  manner  similar  to  hand-made  bricks  (p.  333). 
The  burning  may  be  effected  in  any  kiln  suitable  for  firebricks, 
but  the  greatest  possible  care  must  be  taken  to  avoid  a  shortage 
of  air.  If  there  is  a  reducing  atmosphere  in  the  kiln  the  iron 
compounds  in  the  bauxite  will  be  reduced,  and  bricks  of  very 
low  heat  resistance  will  be  produced.  Bauxite  shrinks  greatly 
during  the  burning,  and  if  this  is  carried  out  at  too  low  a 
temperature  the  bricks  may  shrink  badly  when  in  use.  The 
great  shrinkage  makes  it  very  difficult  to  produce  bauxite  bricks 
of  a  uniform  size,  and  the  men  engaged  in  emptying  the  kilns 
must  be  instructed  to  sort  them  carefully  with  the  aid  of 
gauges.  Unless  this  is  done  it  will  be  impossible  to  keep  the 
joints  in  the  brickwork  uniform. 

Neutral  bricks  are  those  which  are  not  affected  by  either  basic 
substances,  such  as  lime  or  magnesia,  or  by  acid  substances,  such 
as  clay  or  silica.  They  are  more  costly  than  other  bricks,  and 
are  only  used  for  purposes  for  which  the  others  are  unsuitable. 

The  majority  of  neutral  bricks  are  made  of  chromite,  and 
are  composed  of  chromium  oxide  with  some  iron  oxide.  As 
this  material  has  no  inherent  binding  power,  it  is  usually 
mixed  with  twice  its  weight  of  fireclay.  Chromic  bricks  are 
made  in  the  same  manner  as  ordinary  firebricks,  though  some 
makers  prefer  to  compress  them  in  powerful  presses  instead 
of  moulding  them  by  hand. 

Several  other  materials  are  made  into  bricks  for  special 
purposes,  but  their  use  is  too  limited  to  warrant  their  description 
in  the  present  volume. 


INDEX 


O-ORTHO- SILICATE,  51 

Abrasion,  resistance  to  (bricks),  380 
Accelerated  tests,  121 
Accrington,  342 
Adhesion,  1,  229 

between  concrete  and  steel,  255 

of  cement  to  metal,  144 

of  concrete  to  metal,  217,  218 
Adie's  machine,  135 
Adulterants  in  cement,  38,  102 
Aeration,  74 
Agglomerate  clay,  311 
Aggregate,  146 

grading,  156 

measuring,  168 

washing,  154 
Aggregates  for  concrete,  147 

for  reinforced  concrete,  209 
testing,  277 

Air,  exposure  of  cement  to,  92 
Air  separator,  105 
Alite,  50,  52 
Alkali-waste,  18 
Alkalies,  73 

in  clays,  308 
Alligating,  195 
Alluvial  clay,  6,  7,  311 
Alum,  92 
Alumina,  75 

and  lime,  reactions  between,  69 

effect  of  heat  on,  43 

free,  307 

free,  in  cement,  43,  90 

in  cements,  69,  70 

lime  and  silica,  reactions  between, 
71 

^silica  ratio,  65 
Aluminate  theory,  71 
Aluminates,  54,  69,  90 
Aluminium  chloride,  92 
sulphate,  92 
Aluminosilicates,  47,  48,  55,  74,  78,  83, 

85,  87,  89,  90,  93 
Aluminosilicic  acid,  5,  39,  55 
American  Standard  Specification,  121 
Analysis  of  clay,  321 
Anchored  spirals,  223 
Apparent  density,  99,  381 
Aqueducts,  242 


Arches,  241 

Architects'  Institute  recommendations, 
211 

Archless  kiln,  354 

Argillaceous  limestone,  5 

Artificial  aggregates,  149 

Asch,  W.  &  D.,  87,  93,  112,  266 

Asch's  theory,  40,  55,  61,  65,  267 

Ash,  proportion  of,  110 

Ashes  as  aggregate,  151 

Associated  Portland  Cement  Manufac- 
turers, Limited,  154,  155,  157,  169 


/3-ORTHO-SILJCATE,  51 

Bach,  von,  283 
Baked  bricks,  370 
Ball  clays,  311 
mills,  105 

Ballast,  148,  149,  193,  258 
Barker  and  Hunter,  183 
Bars  for  reinforcement,  219 
smooth,  217 
with  wings,  222 
Basic  bricks,  401 
Batch  mixers,  176 
Bauschinger's  method,  124 
Bauxite  bricks,  402 
Beam,  continuous,  216 

loaded,  215 

Beams,  216,  218,  224,  225,  233,  248 
Belgian  cement,  13,  31 

kilns,  351 
Belite,  50,  51 
Bending  moments,  216,  218 

strength,  142 
Berry,  H.  C.,  284 
Binary  silicates,  69 
Binne,  C.  F.,  288 
Binns  and  Makeley,  290 
Bins,  25 
Bitumens,  200 
Blaese,  C.  von,  265 
Blast  furnace  slag,  17 
Block-making  machine,  251 
Blocks,  248 
Blount,  B.,  101 
Blowing  in  bricks,  295,  369 

of  cement,  94,  104,  116,  119 

D  D  2 


404 


INDEX 


Blowing  of  concrete,  150 

Blue  bricks,  289,  372 

Boats,  252 

Bonna  system,  242 

Boracic  acid,  92 

Borax,  92 

Boudouard,  67 

Boulder  clays,  294 

Bradley  and  Craven  stiff-plastic  brick 

machine,  340 

Breaking  strength  of  briquettes,  137 
Brickbats,  377 
Brick  clays,  286 

earth,  311,  326 

Brickmaking,  methods  of,  319 
Bricks,  286 
basic,  401 
bauxite,  402 
chromic,  402 
Fletton,  150,  210 
for  aggregate,  149 
glazed,  359 
magnesite,  401 
neutral,  401,  402 
properties  of,  376 
siliceous,  395 
Bridges,  240 

British  Concrete-Steel  Co.,  212 
British  Fire  Prevention  Committee.  259 
British  Reinforced  Concrete  Engineer- 
ing Co.,  Ltd.,  212 

British  Standard  Specification,  97,  98, 
103,  107,  109,  111,  118,  125,  127,  129, 
131,  132,  133,  135,  137,  139,  140,  147, 
212 

Brittleness  of  bricks,  381 
Brown,  H.  P.,  270 
Buff  bricks,  290 
Buhrer's  kiln,  357 
Builders'  and  Contractors'  Plant,  Ltd., 

176 

Building  blocks,  248 
Bulging,  221 
Bullnoses,  378 
Burning,  33 

bricks,  344 
changes  in,  363 
cement,  23,  68 
Burrs,  305,  377 
Butler,  D.  B.,  104,  108,  116,  122,  137 


CALCAREOUS  SANDS,  159 

Calcined  clay,  43,  46,  47,  48,  88,  367 

Calcium  aluminates,  47,  52,  54,  69,  70, 

71 

aluminosilicates,  47,  55 
carbonate.     See  Chalk,  Lime- 
stone and  Lime, 
chloride,  92,  93 


Calcium  ferrite,  262 

meta-silicate,  67 
mono-silicate,  67 
ortho-silicate,  51,  89 
phosphate,  6 
silicates,  47,  52,  91 
sulphate,  67,  75,  76,  92,  93,  94 
sulphate,  action  of,  on  cement, 

92 

sulphide,  lo 
Callow,  317 
CaO  :  SiO  ratio,  53 
Candlot,  E.,  92,  266 
Cantilever  wall,  220 
Capital  required  for  bricks,  319 
Carbon  dioxide,  action  of,  91,  94 
Carbonaceous  matter  in  clays,  310 
Cart  for  concrete,  188 
Celite,  50,  51 
Cement,  1 

clinker.     See  Clinker. 

coatings,  199 

for  concrete,  147 

for  reinforced  concrete,  210 

hardened,  constituents  of,  90 

in  concrete,  165 

manufacture,  20 

mixture,  effect  of  heat  on,  45 

sand  bricks,  395,  398 

sand  tests,  138 

testing,  96 
Cements,  organic,  1 
Centering,  184 

striking,  193 
Chalcopyrite,  310 
Chalk,  1—3 

and  clay,  reactions  between,  46 

effect  of  heat  on,  44 

in  bricks,  291,  373 
Chamber  kilns,  355 
Changes  to  bricks  in  burning,  358 
Chapman,  C.  M.,  205 
Checking  mixture  of  concrete,  169 
Chemical  changes  in  cements,  39 
Chemical  composition,  98 
Chicago  cube  mixer,  177 
Chimneys  of  concrete,  246 
China  clay,  5,  314 
Chromic  bricks,  402 
Cinders  for  reinforced  concrete,  209 
Clamps,  344 
Clark,  R.  G.,  262 
Clay  and  lime,  reactions  between,  46,  57 

effect  of  heat  on,  40,  366 

preparation  of,  325 
Clays,  1,  2,  5,  39,  55,  286 

action  of  heat  on,  304 

occurrence  of,  12 

plastic,  296 

various,  315 


INDEX 


405 


Cleaning  the  surface  of  concrete,  195 
Climates,  effect  of,  on  setting,  81 
Clinker,  26,  30,  50,  80,  118 
as  aggregate,  151 
bricks,  305,  377,  397 
physical  properties  of,  80 
Clots,  339 
Chinches,  312 
Coal  bunkers,  243 
Cobb,  J.  W.,  66,  67,  69,  72,  76,  77 
Coefficient  of  elasticity,  216 
Coffer-dams,  192 

Coignet,  Edmond,  Limited,  213,  231 
pile,  245 
pipe,  242 

reinforcement,  230 
Coke-breeze  aggregate,  150,  258 

for  reinforced  concrete,  209 
in  clays,  299 

Colloidal  material  in  clay,  301 
silica,  84 
substance,  82 
Colloids,  83 

Colour  of  brick  clays,  287 
of  bricks,  371,  379 
of  terra- cotta,  371 
Colouring  concrete,  196 
Column  base,  224 
Columns,  215,  221 

testing,  279 

Combined  water  in  clay,  367 
Commercial  specifications,  213 
Compactness,  145 
Components  of  concrete,  146 
Composition  of  bricks,  376 

of  cements,  limits  of,  61,  62 
of       concrete      mixtures, 

checking,  169 
of  Portland  cement,  98 
Compression  bar,  228 
Compressive  strength,  126,  224 
Concrete,  146 
blocks,  249 

Institute,  recommendations,  211 
placing,  187 
preparation  of,  162 
reinforced,  206 
spading,  189 
testing,  277 
Conduits,  242 
Cones,  Seger,  360 
Conglomerate  clays,  312 
Considere  Construction  Co.,  Ltd.,  212 
pile,  247 
system,  222 
Consistency  of  concrete,  170 

of  paste  for  bricks,  332 
Continuous  kilns,  347 
Contraction  of  cement,  120 
Cooling  clinker,  26 


Core,  metallic,  223 

Cores,  368 

Corrosion,  electrolytic,  270 

of  reinforced  concrete,  269 
resistance  to,  262 
Costs,  217,  324 
Cracking  (bricks),  369 
of  cement,  116 
of  concrete,  150,  195,  273 
Cracks,  236,  272 

Crocodile  surfaces  of  concrete,  195 
Crozzles,  305,  377 
Crushing,  25 

machinery,  327 

strength,  214,  278 

strength  of  bricks,  385 

strength  of  concrete,  254 

tests,  278 
Crystallisation,  82 
Crystals,  54 

formation  of,  49 

in  cement,  50 
Cushmann,  107 
Cutters,  370,  376 


DAMP-PROOFING  CONCRETE,  197 
"  Darrprobe,"  123 
Day,  67 

Day  and  Shepherd,  51 
Day,  Shepherd  and  Wright,  53 
Dead- burnt  lime,  33 
loads,  213 

loads,  equivalent,  214 
Deflection  under  load,  284 
Dense  surfaces  of  concrete,  195 
Density,  apparent,  99,  145 

of  bricks,  381 
Desch,  C.  H.,  50 
Deval's  test,  123 
"  Diamond  "  stretcher,  378 
Di-calcium  aluminate,  52,  70,  90 

silicate,  52 
Dinas  bricks,  399 
Discoloration  of  concrete,  194,  273 
Ditch  cutters,  323 
Dittler,  E.,  60,  79 
Docks,  242 
Doetler,  C.,  68,  79 
"  Dog-tooth  "  stretcher,  378 
Dolomitic  limestone,  5 
Dormann,  0.,  65,  68 
Down-draught  kilns,  346 
Drift  clays,  312 
Dry  concrete  mixture,  170 

dust  process,  343 

process,  24,  27 
Drying,  24 

bricks,  304,  335,  362 

changes  in,  363 


406 


INDEX 


Drying  clays,  338 
Durability  of  bricks,  386 

of  concrete,  271 
Dye-test,  84 

Dyes  absorbed  by  clays,  301 
Dynamo  beds,  248 


EARTHQUAKE  tremors,  resistance  to,  200 
Edge-runner  grinding  mill,  328 
Efflorescence,  195,  273,  379,  387 
Elasticity  coefficient.  216 
Electrical  conductivity  of  cement  mixes, 

60 

Electrolytic  corrosion,  270 
Emperger,  F.  von,  279 
Encastre,  216 
Engine  beds,  248 
Engineering  bricks,  373,  377 
Expanded  metal,  248 

Metal    Company,    Limited, 

213 

metal  system,  236 
Expansion  of  cement,  94,  116,  125 
of  coke-concrete,  150 


FACING  BKICKS,  343,  362 
concrete,  197 
with  terra-cotta,  258 
Factors  of  safety,  214 
Faija,  H.,  112,  122,  131 
Faija's  mechanical  gauger,  131 
Failure,  causes  of,  253,  274,  275 
Fat  clays,  312 

.lime,  33 

Fawcett  stiff-plastic  brick  machine,  341 
Feichtinger,  87 
Felite,  50,  52 
Felspar,  370 

Felspathic  matter,  effect  of,  72 
Feret,  M.,  226 
Ferrates,  90 
Ferric  oxide,  75 
Ferrocrete,  206 
Ferro-silicates,  74,  90 
Ferrous  carbonate,  289 
compounds,  74 
sulphide,  75 

Filling  in  the  surface  of  concrete,  196 
Final  set,  90,  110 
Fineness,  68,  103 
Fire  Prevention  Committee  tests,  150 

resistance  of  concrete,  256 

resistance  ;   standard  classification 

for  floors,  259 
Firebricks,  305,  362,  375,  395 

durability  of,  393 

selection  of,  389 

silica,  395,  399 


Fireclays,  312,  313 
Fish  glue,  1 

Fletton  bricks,  150,  210 
Flints,  3 

for  aggregate,  149 
Floor  slabs,  248 
Floors,  214,  215,  216,  225,  231,  248 

concrete,  210 

testing,  279 
"  Flour,"  103,  106 
Flourometer,  108 
Fluates,  199 

"  Flying"  of  aggregates,  209 
Forms,  184,  185,  186,  191 
Formulae  of  cement,  88 

of  Portland  cements,  57 
Foul  clays,  297,  317 
Foundations  for  machinery,  248 
Free  lime  in  Portland  cement,  45 
Fremy,  89 
French  standard  for  tensile  strength, 

141 

Frey,  O.,  143 

Frog  in  bricks,  333,  336,  378 
Frost,  effect  of,  190,  271 
Fuel  consumption  in  brick  kilns,  354 
Furnace  slag  as  aggregate,  151 
Furnaces,  lining,  389 
Fusible  clays,  313 
Fusion,  46,  47,  54,  68,  69,  70,  80 


7-CmTHO-SILICATE,  51 

Canister,  313 

bricks,  399 

Garden  ornaments,  252 
Gary,  107,  112 

Gas  Engineers,  Institution  of,  389 
Gas-fired  kilns,  358 
Gate  posts,  252 
Gauging,  81,  113,  115,  131 
Gault  clay,  6,  313 
Gee,  W.  J.,  108 
Gel,  82,  83 
German  standard  for  tensile  strength, 

141 

Standard  Rules,  121 
Girders,  224 

Glassy  matter,  72.  74,  80 
Glazed  bricks,  359,  379 
Gliding,  resistance  to,  145 
Goldbeck,  A.  T.,  284 
Goreham,  process  of,  22 
Graded  sands,  160 
Grading,  261 

aggregates,  153,  253 
Grains,  size  of,  68 
Granite,  148 
Granolithic  facings.  197 
Granulation  of  slag.  37 


INDEX 


407 


Grappier  cement,  34,  266 
Grappiers,  34,  52 
Green  bricks,  344 

concrete,  196 
Grey  bricks,  291 
stocks,  377 
Grinding,  23,  118 
object  of,  47 
Grizzles,  377 
Grog,  314 
Grout,  170 
Guthrie's  kiln,  351 
Gypsum,  76,  93,  94 


HACKS,  335 

Hair-lines  in  concrete,  195 
"  Half-moon  "  stretcher,  378 
Hand-made  bricks,  333 
Hard  materials,  treating,  23 
Hardening,  109 

chambers  for  bricks,  398 
changes  in,  81 
of  concrete,  192 
Hardness  of  bricks,  380 
Haulage,  324 
Hearts  in  bricks,  368 
Heat,  action  of,  on  clays,  40,  304 
on  limestone,  44 
on  silica,  43 
conductivity,  258 
development  of,  in  setting,  86 
effect  of,  on  alumina,  43 
on  bricks,  366 
on     cement     mix,    69, 

79 

on  lime  and  silica,  66 
resistance  to,  by  bricks,  388 
Heating,  insufficient,  372 
Heavy  bricks,  382 

concrete  work.  210 
Hennebique  column,  222 

base,  224 
F.,  221,  227 
piles,  246 
stirrups,  227,  228 
system,  212,  226,  229,  230, 

234,  245 
Herold,  K.,  60 
Hillebrand,  73 
Hoffmann  kiln,  24,  46,  348 
Horizontal  draught  kilns,  347 
Hot  water  tests,  120 
Hubbard,  107 
Humphrey,  R.  L.,  268 
Hydraulic  cements,  1,  2 

lime,  1,  2,  12,  13,  77,  86,  260 
burning,  33 
concrete,  147 
manufacture  of,  32 


Hydraulic  lime,  tensile  strength  of,  142 

modulus,  64 
Hydraulite,  2 
Hydro-silicates,  85 


IMPURITIES,  3,  84 

effect  of,  72 

in  cement,  52 

in  chalk,  21,  46 

in  clays,  305,  307 
Inclined  members,  222 
Indented  bars,  219,  235 
Inert  material  in  concrete,  146 
Initial  set,  81,  88,  90 
Insoluble  residue,  99 
Integral  waterproofing,  200 
Inter-reactions  in  cements,  72 
Iron  compounds,  74 
in  clays,  309 

in  cement,  74 

ore,  18 

ore  waste,  18 

Portland  cement,  36 
Irregularity  in  bricks,  378 


JESSER,  L.,  60 

Johnson,  R.,  Clapham  and  Morris,  Ltd., 

239,  240 

Johnson's  kiln,  22 
Jones,  H.  R.,  255 


KAHN  SYSTEM,  212 

trussed  bar,  217,  231,  232 
Kallauner,  265 
Kaolin,  314 
Kaolinite,  306 
Keedon  bar,  229,  234 
Keene's  cement,  1 
Kentish  rag,  139 

specific  gravity  of,  102 
Kibblers,  328 
Kiefer,  H.  E.,  119 
Kiln,  Hoffmann,  24 

Johnson's,  22 

rotary,  25 

shaft,  23 
Kilns,  23,  33,  46,  72,  76,  79,  110,  345 

changes  occurring  in,  366 
Kloes,  J.  A.  van  der,  267 
Kiihl,  H.,  117 


LAMINATED  clays,  314 

Lateral  ties,  221 

Lavas,  2,  14 

Lean  clays,  297,  314 

Lean  mixture  concrete,  167 


408 


INDEX 


Le  Chatelier,  50,  52,  55,  101,  120,  121, 

285 

Le  Chatelier' s  test,  124 
Leicester  red  bricks,  293 
Lias  limestone,  4,  14 
Liassic  marls,  11 
Light  bricks,  382 
Lime,  1,  2 

action  of  heat  on,  45 

alumina  and  silica,  reactions  be- 
tween, 71 

-alumina  ratio,  53 

and  clay,  reactions  between,  46,  57 

and  silica,  effect  of  heat  on,  66 
reactions  between,  66 

carbonated,  91 

carbonation  of,  94 

compounds,  3 
in  clay,  307 

concrete,  147,  167,  214 

effect  of,  on  setting,  83 

(free)  in  cement,  45,  117,  118,  119 

hydraulic.     See  Hydraulic  lime. 

minimum  proportion  of,  65 

sand  bricks,  395 
Limestone,  1,  2,  4,  165 

action  of  heat  on,  44 

in  bricks  clays,  295,  369 
Limestones  for  aggregate,  149 
Lindner,  107 

Lines  of  stress  in  beam,  233 
Lintels,  250 
Litre  weight,  99 
Live  loads,  213 
Loads,  calculating,  213,  215 
heavy,  228 
in  buildings,  213 
varying,  215 
Loading  tests,  279 
Loams,  8,  293,  326 
London  clay,  315 
L.C.C.  rules,  219 
London  stocks,  345 
Loss  in  manufacture  of  bricks,  362 

of  shape  (bricks),  370 

on  ignition,  99 


MAGAZINES,  243 

Magnesia,  5,  46,  57,  73,  99,  117,  265 

compounds,  reactions  of,  266 

effects  of,  73 
Magnesite  bricks,  401 
Magnesium  chloride,  93 

compounds  in  clays,  308 
Malm  bricks,  290 
Malms,  292,  376 
Manufacture,  changes  in,  39 
Marcasite,  310 
Marine  deposited  clays,  315 


Maritime  work,  267 
Marls,  5,  9,  292,  326 

for  brickmaking,  10 

for  cement  manufacture,  10 

of  Staffordshire,  289 

red,  293 
Materials  for  cements,  1 

selection  of,  323 
Meal,  25 
Mechanical  bond,  228 

mixers,  176 
Medium  mixture  concrete,  169 

setting,  110 

Melting     point     and     electrical     con- 
ductivity, 61 

Membrane  waterproofing,  200,  203 
Meta-silicates,  67,  82 
Methods  of  cement  manufacture,  20 
Michaelis,  W.,  104,  106 
Microscopical  examination,  49 

study  of  cement,  84 
Midland  marls,  315 
Mild  clays,  297 
Mills,  25 
Millstones,  105 
Mineral  matter,  effect  of,  72J 
Minerals  in  clay,  44 
Mining,  323 
Mixer  for  clay,  331 
Mixers,  25,  176—183 
Mixing  clay  for  bricks,  331 

concrete,  173 
Mixture  theory,  48 
Moisture,  exposure  to,  92 
Mono-silicate,  67 
Morsch,  283 
Mortar,  5 

mill,  332 

tensile  strength  of,  142 
Moss  bar,  234,  235 
Mouchel,  L.  G.,  and  Partners,  212 

hollow  pile,  245 
Moulding  bricks,  333 
Moulds,  184 
Muds,  2,  7,  9 
Mundic,  310 


NATURAL  cement,  2,  12,  77 

manufacture  of,  29 
Neat  cement,  129 
Neat  tensile  test,  138 
Netting,  239 

Network  for  reinforcing,  22 
Neutral  bricks,  401,  402 
Newberry  Bros.,  53,  73,  74 
Newcastle  kilns,  347 
Nodules,  6,  13 
Nontronite,  288 
Norton,  C.  L.,  258,  260 


INDEX 


409 


OILS  in  concrete,  204 
Ordinary  concrete  mixture,  170 
Ore,  18 

Ornamental  bricks,  378 
Ormim,  Van,  284 
Ortho-silicates,  51,  89 
Ostwald,  W.,  112 
Overburden,  317 
Over-heating,  33 
Overlimed  cement,  104 
Overloaded  columns,  221 
Oxy-chloride  cements,  1 
Oxy-phosphate  cements,  1 


PAINT  applied  to  concrete,  199 
Pan  mill,  332 
Paving  blocks,  248 
bricks,  377 
Paviours,  377 
Parian  cement,  1 
Pebbles,  148 
Peeling,  195,  196 

of  bricks,  387 
Pelinite,  306 
Permeability  of  bricks,  384 

of  concrete,  261,  265 
tests  of,  284 
Permean  marls,  11 
Peterborough,  342 
Physical  changes  in  cements,  39 
Piers,  215,  221 
Piles,  concrete,'  243 
Pipes,  concrete,  242 
Pit  props,  252 
Placing  concrete,  187 

in  water,  191 
Plaster  of  Paris,  76,  94 

slag  cements,  36 
Plastic  clays,  296 

methods  of  brickmaking,  333 
"  Plums,"  154 
Plunge  test,  122 
Polished  concrete,  196 
Pontoons,  252 
Popplewell,  255 
Pore  water,  364 
Porosity,  145 

of  aggregates,  158 

of  bricks,  383 

Porous  surfaces  of  concrete,  195 
Portland  cement,  1 

action  of  sea  water  on,  266 

adulterants  of,  38 

manufacture  of,  20 
Potash,  73 
Potassium  carbonate,  92 

compounds  in  clay,  308 
di-chromate,  92 
sulphate,  92 


Potassium  sulphide,  92 
Pozzolana,  1,  2,  15,  35,  42,  77,  78,  88, 
146,  205 

tensile  strength  of,  142 
Pozzolanic  sands,  159 
slag,  17 

cements,  36 
Practical  test,  96 
Preheating  bricks,  351 
Preparation  of  concrete,  162 
Properties  of  concrete,  254 
Proportions,  30,  62,  68,  71 

for  beams,  etc.,  224 

for  reinforced  concrete,  210 

in  concrete  work,  252 

of  clay  and  lime,  12 

of  components  of  concrete,  163 
Pugmill,  330 
"  Pure  clay,"  306 
Purple  bricks,  289,  371 
Pyrites,  6,  290,  309,  370 


QUARRYING,  323 
Quartz,  66,  68 
Quicklime,  5,  33 

manufacture  of,  32 
Quick  setting,  110 

cement,  81,  90,  94,  109 


RACE  in  bricks,  369 
Rafts,  248 

Railway  sleepers,  252 
Ramming,  189 

mechanical,  142 
Rankine,  67 
Ransome  ver  Mehr  Machinery  Company, 

176,  179,  181,  182,  187 
Rasenow,  299 

Rate  of  setting,  82,  92,  95,  109 
Rational  analysis,  306 
Raw  materials  for  bricks,  286 
Raw  meal,  25 
Reading  clays,  315 
Red-burning  clays,  288 
Reducing  atmosphere,  76,  372 
Refractoriness  of  bricks,  388 
Refractory  clays,  315 
Re-heating,  103 
Reinforced  concrete,  206 

Metal,  Limited,  223 
Re-pressing  bricks,  343 
Reservoirs,  242 
Resistance  to  abrasion  of  bricks,  380 

to  corrosion,  262 

to  shocks,  260 

to  strong  acids  of  bricks,  388 
Retaining  walls,  220 
Retardation  of  setting,  92 


410 


INDEX 


Rib  mesh  reinforcement,  237 
Rich  mixture  concrete,  167 
Richardson,  51 
Roads,  concrete,  250 
Rock  cement,  13 

manufacture  of,  29 

tensile  strength  of,  142 
Rock  clays,  315 
Rohland,  P.,  92 
Roman  cement,  1,  13,  31,  77,  90 

tensile  strength  of,  142 
Roof,  216 
Ropeways,  324 
Ross,  190 

Rotary  kiln,  25,  46,  72 
Rough  stocks,  377 
Roughness,  195 
R.I.B.A.  Committee,  211,  218,  257,  279, 

283 

standard  for  bricks,  378 
"  Ruabon  "  kiln,  356 
Ruabon  terra-cotta  clay,  293 
"  Rubbers,"  370,  376 


SAFE  load  for  concrete,  214 

for  lime-concrete,  214 
Safety  factors,  214 
Salter  tensile  test  machine,  136 
Salts,  effect  of,  113 

soluble,  370 
Sampling,  98 
Sand,  146,  150,  159 

cement,  29 

cementitious,  193 

for  reinforced  concrete,  209 

measuring,  168 

moulding,  334 

standard,  139 

testing,  277 

Sandstones  for  aggregate,  149 
Sandy  clays,  294,  316,  326 
Santorin  earth,  15 
Schmidt  and  Unger,  54 
Schule's  machine,  143 
Schuljatschenko,  267 
Scotch  kilns,  345 
Scott's  cement,  34 
Scum,  195,  293,  309,  379,  387 
Scummed  bricks,  291 
Sea  sand,  209 

water,  93 

action  of,  264 
Seasoning,  119 
Seccotine,  1 
Seger  cones,  360 
Selenitic  cement,  34 
Semi-dry  process  of  brickinaking,  333, 
341 


Semi-plastic  methods  of  brickmaking. 

333,  339 
Septaria,  12 

in  bricks,  369 
Setting  of  cement,  76 

of  concrete,  192 
rate  of,  81,  82,  109 
retardation  of,  92 

Sewage,  action  of,  on  concrete,  263 
Sewell,  J.  S.,  270 
Sewers,  242 
Shaft  kiln,  23 
Shakes,  377 
Shales,   5,   8,  55,   286,  292,  314,  316, 

326 

for  aggregate,  149 
occurrence  of,  12 
Shapes  of  bricks,  378 
Shaping  the  clay,  333 
Shear  bar,  218 

diagram,  215 
members,  219,  233 
reinforcement,  225 
resistance  to,  145,  235 
tests,  254 

Shearing  strength,  144,  254,  255 
stress  of  concrete,  226 
Shelling  of  bricks,  387 
Shepherd,  67,  69 
Shivers,  377 

Shocks,  resistance  to,  260 
Shrinkage  measurements,  360 

of  clay,  301,  302,  363,  389 
of  concrete,  195 
water,  364 
ShufiEs,  377 
Shuttering,  184 
Shutters,  184 
Siderite,  289 
Sifting,  105 
Silica,  action  of  heat  on,  43 

alumina     and     lime,     reactions 

between,  71 

and  lime,  reactions  between,  66 
bricks,  399 
colloidal,  84 
firebricks,  395,  399 
free,  90,  306 
in  Portland  cement,  67 
Silicates,  51,  52,  67,  69,  72 

containing  iron,  309 
Siliceous  bricks,  395 
Sillimanite,  42 
Silos,  25,  243 
Silt,  316 

Size  of  sand  particles,  160 
Sizes  of  bricks,  378 

of  particles  in  aggregate,  151,  153 
Slabs,  216,  248 
testing,  279 


INDEX 


411 


Slag  cements,  78 

manufacture  of,  35 
tensile  strength  of,  142 
Slag,  specific  gravity  of,  102 
Slags,  2,  16,  51,  67,  76,  78,  258 
Slate  waste,  8 
Slates,  316 
Sleepers,  252 

Sliding  of  reinforcement,  144 
Slipping  of  stirrups  or  bars,  233 
Slop  moulding,  334 
Slow  setting,  110 

cements,  86,  92,  109,  269 
Slurry,  21 

Smith,  T.  L.,  Co.,  Ltd.,  176,  178,  179 
"Smoking  "  bricks,  352 
Soaps  in  concrete,  204 
Soda,  73 
Sodium  carbonate,  92 

compounds  in  clay,  308 
sulphate,  92 
Soil,  316 
Sol,  83 
Solid  solution,  70,  86 

theory,  48 
Soluble  salts,  370 
silica,  43 
Soundness  of  cements,  116,  117 

tests,  119 

Spade  for  concrete,  190 
Spading,  195 
Spalling  (concrete),  195 

(bricks),  369 
Spans,  216 
Specific  gravity,  100 

of  bricks,  381 

Specification  for  firebricks,  391 
Specifications,  commercial,  213 
Spiral  bars,  236 

reinforcement,  242 
Spissograph,  114 
Splintering,  209 
Spofforth,  226 
Spots  on  bricks,  370 
Squints,  378 
Staffordshire  bricks,  372 

kiln,  355 
Stairways,  251 
Stanchions,  221,  224 
Standard  mixture  concrete,  167 
sand, 139 

specifications  for  cements,  97 

Stanger,  W.  H.,  101 

Stationary  kilns,  72 

Steam,  effect  of,  92 

navvies,  323 

Steel  for  reinforcement,  211 
in  concrete,  206 
testing,  277 
Steelcrete,  206 


Steinbriick,  131 

-Schmelzer  machine,  133 
Steps,  251 

Stiff-plastic  process,  333,  339 
Stirrups,  218 

Hennebique,  227,  228 
Stocks,  377 
Stones,  148 
Stony  clay,  294 
Storage,  25 

Straight  line  formula,  217 
Strength  of  bricks,  384 
of  cement,  90 
of  concrete,  254,  271 
of  mixtures,  104 
to  age,  ratio  of,  227 
Stress,  distribution  of,  239 
lines  of,  215 
shearing,  255 
Stresses,  internal,  216 

working,  213,  216 
Striking  centering,  193 
String  course  bricks,  378 
Strong  clays,  297,  317 
Submarine  work,  265 
Suffolk  bricks,  370 
Sulphates,  67,  75,  117,  211,  265 
effect  of,  93 
in  clay,  307 
Sulphide  of  iron,  309 
Sulphides,  75 
Sulphur,  258 

compounds  in  concrete,  151 
in  concrete,  150 
tri-oxide,  76,  99 
Surface  clays,  6,  317 
fillings,  196 

treatment  of  concrete,  194 
Swelling,  221 

Sylvester  process  of  damp-proofing,  198 
Systems  of  reinforcement,  217 


TALBOT,  226 

Tamping,  189 
tool,  189 

Tanks,  concrete,  210,  242 

Tar,  199 

Technical  knowledge  needed  by  brick- 
makers,  320 

Teil  lime,  34 

Telegraph  poles,  252 

Temperature,  effect  of,  115 

required  in  burning 
bricks,  374 

Tempering  mill,  332 

Tender  clays,  317 

Tensile  strength,  128,  161 

of  concrete,  254 
of  mixtures,  139 


412 


INDEX 


Tensile  strength  of  slag  cements,  142 
Tension  bars,  225 

diagonal,  226 

Terra-cotta,  architectural,  303 
facing  with,  258 
for  aggregate,  149 
Testing  aggregates,  154,  277 
cements,  96 
clay  for  bricks,  321 
concrete,  277 
Tests,  loading,  279 

of  concrete  columns,  222 
Tetmajor,  128 
Texture  of  bricks,  382 

of  clay,  292 
Thermal  method,  113 
Tiles,  cement,  251 
Till  clay,  317 

Time  of  setting,  81,  88,  112 
Tornebohm,  50 
Tosca,  15 
Tough  clays,  297 
Toughness  of  bricks,  381 
Tournai  cement,  31 
Tramway  standards,  252 
Transverse  bending  strength,  143 
bonds,  221 
strength  of  concrete,   254, 

Trass,  1,  14,  15,  42,  88,  205,  269,  271 

tensile  strength  of,  142 
Tremie,  use  of,  192 
Trials,  359 

Triangle  mesh  reinforcement,  240 
Triassic  clays,  315 
marls,  11 
Tri-calcium  aluminate,  69,  90 

silicate,  52,  67,  69,  90 
Tridymite,  68 
Truss  girder,  225 
Trussed  bar,  231 

Trussed  Concrete  Steel  Co.,  Ltd.,  212 
Tube-mills,  105 
Tuffs,  2,  14 
Tunnel  dryer,  338 
Turneaure,  228 


UNDER-BURNING,  119 
Unsoundness,  116 
Unwin,  128 
Updraught  kilns,  345 


VIBRATION  of  concrete,  256 


Vibrators,  189 
Vibrocel  Co.,  Ltd.,  189,  192 
Vicat  needle,  110,  111 
Vitrifiable  clays,  317 
Voids,  152,  157,  163 

in  sand,  161 
Volcanic  lavas,  2,  14 
tuffs,  2 


WALLS,  concrete,  210 
Water  absorption,  145 

action  of,  on  cement,  85 

combined,  in  brick,  367 

for  concrete,  147 

fresh,  action  of,  on  concrete,  263 

in  bricks,  363 

-mains,  242 

-proofing  concrete,  197 

proportion  of,  in  concrete,  172 

repellents,  204 

-smoking  bricks,  352 

tanks,  242 
Weak  clays,  297 
Weathering,  327 
Web-connection,  228 
Web  members,  233 

reinforcement,  225 
Weight  per  bushel,  99 

per  cubic  foot,  215 
Wet  concrete  mixture,  170 

process,  21,  27,  118 
White  bricks,  290 

burning  clays,  290 
Whittaker  plunger  press,  342 
Wilson  and  Gay  lord,  168 
Wine  musts,  action  of,  on  concrete,  263 
"  Winget  "  concrete  blocks,  250 
Winget    Concrete    Machine    Company, 

176,  180,  251 
Wire-cut  bricks,  335 

process,  337 
Wire  netting,  239 
"  Wolves,"  328 
Woolson,  258 
Wright,  68 


YELLOW  bricks,  290 
clays,  318 


ZSCHOKKE,  299 
Zulkowski,  88,  89 


BRADBURY,    AGNKW,    &    CO.    LI>.,    PRINTERS,    LONDON    AND   TONBR1DGE. 


VAN  NOSTRAND'S 
"WESTMINSTER"  SERIES 

Bound  in  Uniform  Style. 
Fully  Illustrated.        Price  S2.OO  net  each. 

Gas  Engines*    By  W.  J.  MARSHALL,  Assoc.  M.I.Mech.E., 
and  CAPT.  H.  RIALL  SANKEY,  R.E.  (Ret.).    M.Inst.C.E., 
M.I.Mech.E.    300  Pages,  127  Illustrations. 
LIST  OF  CONTENTS  :  Theory  of  the  Gas  Engine.   The  Otto  Cycle.   The 
Two  Stroke  Cycle.     Water  Cooling  of  Gas  Engine  Parts.     Ignition. 
Operating  Gas  Engines.     The  Arrangement  of  a  Gas  Engine  Instal- 
lation.    The  Testing  of  Gas  Engines.     Governing.     Gas  and  Gas 
Producers.     Index. 

Textiles*  By  A.  F.  BARKER,  M.Sc.,  with  Chapters  on  the 
Mercerized  and  Artificial  Fibres,  and  the  Dyeing  of 
Textile  Materials  by  W.  M.  GARDNER,  M.Sc.,  F.C.S. ; 
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Industry,  by  W.  H.  COOK  ;  the  Linen  Industry,  by  F. 
BRADBURY.  370  Pages.  86  Illustrations. 

CONTENTS  :  The  History  of  the  Textile  Industries  ;  also  of  Textile 
Inventions  and  Inventors.  The  Wool,  Silk,  Cotton,  Flax,  etc., 
Growing  Industries.  The  Mercerized  and  Artificial  Fibres  em- 
ployed in  the  Textile  Industries.  The  Dyeing  of  Textile  Materials. 
The  Principles  of  Spinning.  Processes  preparatory  to  Spinning. 
The  Principles  of  Weaving.  The  Principles  of  Designing  and 
Colouring.  The  Principles  of  Finishing.  Textile  Calculations. 
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Goods,  Stuff,  and  Linings  Industry.  The  Tapestry  and  Carpet 
Industry.  Silk  Throwing  and  Spinning.  The  Cotton  Industry. 
The  Linen  Industry  historically  and  commercially  considered. 
Recent  Developments  and  the  Future  of  the  Textile  Industries. 
Index. 

Soils  and  Manures*    By  J.  ALAN  MURRAY,  B.Sc.    367 

Pages.     33  Illustrations. 

CONTENTS  :  Introductory.  The  Origin  of  Soils.  Physical  Proper- 
ties of  Soils.  Chemistry  of  Soils.  Biology  of  Soils.  Fertility. 
Principles  of  Manuring.  Phosphatic  Manures.  Phosphonitro- 
genous  Manures.  Nitrogenous  Manures.  Potash  Manures. 
Compound  and  Miscellaneous  Manures.  General  Manures.  Farm- 
yard Manure.  Valuation  of  Manures.  Composition  and  Manural 
Value  of  Various  Farm  Foods. 


THE    "  WESTMINSTER  "    SERIES 

Coal.  By  JAMES  TONGE,  M.I.M.E.,  F.G.S.,  etc.  (Lecturer 
on  Mining  at  Victoria  University,  Manchester).  283 
Pages.  With  46  Illustrations,  many  of  them  showing  the 
Fossils  found  in  the  Coal  Measures. 

LIST  OF  CONTENTS  :  History.  Occurrence.  Mode  of  Formation 
of  Coal  Seams.  Fossils  of  the  Coal  Measures.  Botany  of  the 
Coal-Measure  Plants.  Coalfields'  of  the  British  Isles.  Foreign 
Coalfields.  The  Classification  of  Coals.  The  Valuation  of  Coal. 
Foreign  Coals  and  their  Values.  Uses  of  Coal.  The  Production 
of  Heat  from  Coal.  Waste  of  Coal.  The  Preparation  of  Coal 
for  the  Market.  Coaling  Stations  of  the  World.  Index. 

Iron  and  Steel.    By  J.  H.  STANSBIE,  B.Sc.  (Lond.),  F.I.C. 

385  Pages.     With  86  Illustrations. 

LIST  OF  CONTENTS  :  Introductory.  Iron  Ores.  Combustible  and 
other  materials  used  in  Iron  and  Steel  Manufacture.  Primitive 
Methods  of  Iron  and  Steel  Production.  Pig  Iron  and  its  Manu- 
facture. The  Refining  of  Pig  Iron  in  Small  Charges.  Crucible 
and  Weld  Steel.  The  Bessemer  Process.  The  Open  Hearth 
Process.  Mechanical  Treatment  of  Iron  and  Steel.  Physical 
and  Mechanical  Properties  of  Iron  and  Steel.  Iron  and  Steel 
under  the  Microscope.  Heat  Treatment  of  Iron  and  Steel.  Elec- 
tric Smelting.  Special  Steels.  Index. 

Timber*     By  J.  R.    BATERDEN,  Assoc.M.Inst.C.E.     334 

Pages.     54  Illustrations. 

CONTENTS  :  Timber.  The  World's  Forest  Supply.  Quantities  of 
Timber  used.  Timber  imports  into  Great  Britain.  European 
Timber.  Timber  of  the  United  States  and  Canada.  Timbers 
of  South  America,  Central  America,  and  West  India  Islands.  Tim- 
bers of  India,  Burma,  and  Andaman  Islands.  Timber  of  the 
Straits  Settlements,  Malay  Peninsula,  Japan  and  South  and 
West  Africa.  Australian  Timbers.  Timbers  of  New  Zealand 
and  Tasmania.  Causes  of  Decay  and  Destruction  of  Timber. 
Seasoning  and  Impregnation  of  Timber.  Defects  in  Timber  and 
General  Notes.  Strength  and  Testing  of  Timber.  "  Figure  "  in 
Timber.  Appendix.  Bibliography. 

Natural  Sources  of  Power.  By  ROBERT  S.  BALL,  B.Sc., 
A.M.Inst.C.E.  362  Pages.  With  104  Diagrams  and 
Illustrations. 

CONTENTS  :  Preface.  Units  with  Metric  Equivalents  and  Abbre- 
viations. Length  and  Distance.  Surface  and  Area.  Volumes. 
Weights  or  Measures.  Pressures.  Linear  Velocities,  Angular 
Velocities.  Acceleration.  Energy.  Power.  Introductory 
Water  Power  and  Methods  of  Measuring.  Application  of  Water 
Power  to  the  Propulsion  of  Machinery.  The  Hydraulic  Turbine. 
(  2  ) 


THE   "WESTMINSTER"   SERIES 

Various  Types  of  Turbine.  Construction  of  Water  Power  Plants. 
Water  Power  Installations.  The  Regulation  of  Turbines.  Wind 
Pressure,  Velocity,  and  Methods  of  Measuring.  The  Application 
of  Wind  Power  to  Industry.  The  Modern  Windmill.  Con- 
structional Details.  Power  of  Modern  Windmills.  Appendices. 
A,  B,C  Index. 

Electric    Lamps,      By    MAURICE    SOLOMON,    A.C.G.I., 

A.M.I.E.E.  339  Pages.  112  Illustrations. 
CONTENTS  :  The  Principles  of  Artificial  Illumination.  The  Produc- 
tion of  Artificial  Illumination.  Photometry.  Methods  of  Testing. 
Carbon  Filament  Lamps.  The  Nernst  Lamp.  Metallic  Filament 
Lamps.  The  Electric  Arc.  The  Manufacture  and  Testing  of  Arc 
Lamp  Carbons.  Arc  Lamps.  Miscellaneous  Lamps.  Compari- 
son of  Lamps  of  Different  Types. 

Liquid  and  Gaseous  Fuels,  and  the  Part  they  play 
in  Modern  Power  Production.  By  Professor 
VIVIAN  B.  LEWES,  F.I.C.,  F.C.S.,  Prof,  of  Chemistry, 
Royal  Naval  College,  Greenwich.  350  Pages.  With  54 
Illustrations. 

LIST  OF  CONTENTS  :  Lavoisier's  Discovery  of  the  Nature  of  Com- 
bustion, etc.  The  Cycle  of  Animal  and  Vegetable  Life.  Method 
of  determining  Calorific  Value.  The  Discovery  of  Petroleum 
in  America.  Oil  Lamps,  etc.  The  History  of  Coal  Gas.  Calorific 
Value  of  Coal  Gas  and  its  Constituents.  The  History  of  Water 
Gas.  Incomplete  Combustion.  Comparison  of  the  Thermal 
Values  of  our  Fuels,  etc.  Appendix.  Bibliography.  Index. 

Electric   Power    and    Traction.     By  F.  H.  DAVIES, 

A.M.T.E.E.  299  Pages.  With  66  Illustrations. 
LIST  OF  CONTENTS  :  Introduction.  The  Generation  and  Distri- 
bution of  Power.  The  Electric  Motor.  The  Application  of 
Electric  Power.  Electric  Power  in  Collieries.  Electric  Power 
in  Engineering  Workshops.  Electric  Power  in  Textile  Factories. 
Electric  Power  in  the  Printing  Trade.  Electric  Power  at  Sea. 
Electric  Power  on  Canals.  Electric  Traction.  The  Overhead 
System  and  Track  Work.  The  Conduit  System.  The  Surface 
Contact  System.  Car  Building  and  Equipment.  Electric  Rail- 
ways. Glossary.  Index. 

Decorative    Glass    Processes.      By    ARTHUR    Louis 

DUTHIE.     279  Pages.     38  Illustrations. 

CONTENTS  :  Introduction.  Various  Kinds  of  Glass  in  Use  :  Their 
Characteristics,  Comparative  Price,  etc.  Leaded  Lights.  Stained 
Glass.  Embossed  Glass.  Brilliant  Cutting  and  Bevelling.  Sand- 
Blast  and  Crystalline  Glass.  Gilding.  Silvering  and  Mosaic. 
Proprietary  Processes.  Patents.  Glossary. 

(  3  ) 


THE   "WESTMINSTER"    SERIES 


Town    Gas   and  its  Uses  for  the  Production  of 
Light,  Heat,  and  Motive  Power.    By  W.  H.  Y. 
WEBBER,  C.E.     282  Pages.     With  71  Illustrations. 
LIST  OF  CONTENTS  :   The  Nature  and  Properties  of  Town  Gas.     The 
History  and  Manufacture  of  Town  Gas.     The  Bye-Products  of 
Coal    Gas    Manufacture.     Gas    Lights    and    Lighting.      Practical 
Gas  Lighting.     The  Cost  of  Gas  Lighting.     Heating  and  Warm- 
ing by  Gas.     Cooking  by  Gas.     The  Healthfulness  and  Safety 
of  Gas  in  all  its  uses.     Town  Gas  for  Power  Generation,  including 
Private  Electricity  Supply.     The  Legal   Relations  of  Gas   Sup- 
pliers, Consumers,  and  the  Public.     Index. 

Electro-Metallurgy.      By    J.    B.    C.    KERSHAW,   F.I.C. 

318  Pages.     With  61  Illustrations. 

CONTENTS  :  Introduction  and  Historical  Survey.  Aluminium. 
Production.  Details  of  Processes  and  Works.  Costs.  Utiliza- 
tion. Future  of  the  Metal.  Bullion  and  Gold.  Silver  Refining 
Process.  Gold  Refining  Processes.  Gold  Extraction  Processes. 
Calcium  Carbide  and  Acetylene  Gas.  The  Carbide  Furnace  and 
Process.  Production.  Utilization.  Carborundum.  Details  of 
Manufacture.  Properties  and  Uses.  Copper.  Copper  Refin- 
ing. Descriptions  of  Refineries.  Costs.  Properties  and  Utiliza- 
tion. The  Elmore  and  similar  Processes.  Electrolytic  Extrac- 
tion Processes.  Electro-Metallurgical  Concentration  Processes. 
Ferro-alloys.  Descriptions  of  Works.  Utilization.  Glass  and 
Quartz  Glass.  Graphite.  Details  of  Process.  Utilization.  Iron 
and  Steel.  Descriptions  of  Furnaces  and  Processes.  Yields  and 
Costs.  Comparative  Costs.  Lead.  The  Salom  Process.  The  Betts 
Refining  Process.  The  Betts  Reduction  Process.  White  Lead  Pro- 
cesses. Miscellaneous  Products.  Calcium.  Carbon  Rhulphide. 
Carbon  Tetra-Chloride.  Diamantine.  Magnesium.  Phosphorus. 
Silicon  and  its  Compounds.  Nickel.  Wet  Processes.  Dry 
Processes.  Sodium.  Descriptions  of  Cells  and  Procco.,e;.  Tin'. 
Alkaline  Processes  for  Tin  Stripping.  Acid  Processes  for  Tin 
Stripping.  Salt  Processes  for  Tin  Stripping.  Zinc.  Wet  Pro 
cesses.  Dry  Processes.  Electro-Thermal  Processes.  Electro 
Galvanizing.  Glossary.  Name  Index. 

Radio-Telegraphy.    By  C.  C.   F.  MONCKTON,   M.I.E.E. 

389  Pages.  With  173  Diagrams  and  Illustrations. 
CONTENTS  :  Preface.  Electric  Phenomena.  Electric  Vibrations. 
Electro-Magnetic  Waves.  Modified  Hertz  Waves  used  in  Radio- 
Telegraphy.  Apparatus  used  for  Charging  the  Oscillator.  The 
Electric  Oscillator  :  Methods  of  Arrangement,  Practical  Details. 
The  Receiver  :  Methods  of  Arrangement,  The  Detecting  Ap- 
paratus, and  other  details.  Measurements  in  Radio-Telegraphy. 
The  Experimental  Station  at  Elmers  End  :  Lodge-Muirhead 
System.  Radio  -  Telegraph  Station  at  Nauen  :  Telefunken 
System.  Station  at  Lyngby  :  Poulsen  System.  The  Lodge- 

(  4  ) 


THE    "WESTMINSTER"    SERIES 

Muirhead  System,  the  Marconi  System,  Telefunken  System,  and 
Poulsen  System.  Portable  Stations.  Radio-Telephony.  Ap- 
pendices :  The  Morse  Alphabet.  Electrical  Units  used  in  this 
Book.  International  Control  of  Radio-Telegraphy.  Index. 

India-Rubber  and  its  Manufacture,  with  Chapters 
on  Gutta-Percha  and  Balata.  By  H.  L.  TERRY, 
F.I.C.,  Assoc.Inst.M.M.  303  Pages.  With  Illustrations. 

LIST  OF  CONTENTS  :  Preface.  Introduction  :  Historical  and 
General.  Raw  Rubber.  Botanical  Origin.  Tapping  the  Trees. 
Coagulation.  Principal  Raw  Rubbers  of  Commerce.  Pseudo- 
Rubbers.  Congo  Rubber.  General  Considerations.  Chemical 
and  Physical  Properties.  Vulcanization.  India-rubber  Planta- 
tions, india-rubber  Substitutes.  Reclaimed  Rubber.  Washing 
and  Drying  of  Raw  Rubber.  Compounding  of  Rubber.  Rubber 
Solvents  and  their  Recovery.  Rubber  Solution.  Fine  Cut  Sheet 
and  Articles  made  therefrom.  Elastic  Thread.  Mechanical 
Rubber  Goods.  Sundry  Rubber  Articles.  India-rubber  Proofed 
Textures.  Tyres.  India-rubber  Boots  and  Shoes.  Rubber  for 
Insulated  Wires.  Vulcanite  Contracts  for  India-rubber  Goods. 
The  Testing  of  Rubber  Goods.  Gutta-Percha.  Balata.  Biblio- 
graphy. Index. 

The  Railway  Locomotive,  What  It  Is,  and  Why  It  is 
What  It  Is.  By  VAUGHAN  PENDRED,  M.Inst.M.E., 
Mem.Inst.M.I.  321  Pages.  94  Illustrations. 

CONTENTS  :  The  Locomotive  Engine  as  a  Vehicle — Frames.  Bogies. 
The  Action  of  the  Bogie.  Centre  of  Gravity.  Wheels.  Wheel 
and  Rail.  Adhesion.  Propulsion.  Counter-Balancing.  The  Loco- 
motive as  a  Steam  Generator — The  Boiler.  The  Construction  of  the 
Boiler.  Stay  Bolts.  The  Fire-Box.  The  Design  of  Boilers. 
Combustion.  Fuel.  The  Front  End.  The  Blast  Pipe.  Steam 
Water.  Priming.  The  Quality  of  Steam.  Superheating.  Boiler 
Fittings.  The  Injector.  The  Locomotive  as  a  Steam  Engine — 
Cylinders  and  Valves.  Friction.  Valve  Gear.  Expansion.  The 
Stephenson  Link  Motion.  Walschaert's  and  Joy's  Gears.  Slide 
Valves.  Compounding.  Piston  Valves.  The  Indicator.  Ten- 
ders. Tank  Engines.  Lubrication.  Brakes.  The  Running  Shed. 
The  Work  of  the  Locomotive. 

Glass  Manufacture.  By  WALTER  ROSENHAIN,  Superin- 
tendent of  the  Department  of  Metallurgy  in  the  National 
Physical  Laboratory,  late  Scientific  Adviser  in  the  Glass 
Works  of  Messrs.  Chance  Bros.  &  Co.  280  Pages.  With 
Illustrations. 

CONTENTS:  Preface.  Definitions.  Physical  and  Chemical  Qualities, 
Mechanical,  Thermal,  and  Electrical  Properties.  Transparency 

(  5  ) 


THE   "WESTMINSTER"    SERIES 

and  Colour.  Raw  materials  of  manufacture.  Crucibles  and 
Furnaces  for  Fusion.  Process  of  Fusion.  Processes  used  in 
Working  of  Glass.  Bottle.  Blown  and  Pressed.  Rolled  or 
Plate.  Sheet  and  Crown.  Coloured.  Optical  Glass :  Nature 
and  Properties,  Manufacture.  Miscellaneous  Products.  Ap- 
pendix. Bibliography  of  Glass  Manufacture.  Index 

Precious  Stones.    By  W.  GOODCHILD,  M.B.,  B.Ch.    319 
Pages.    With  42  Illustrations.    With  a  Chapter  on 
Artificial  Stones.    By  ROBERT  DYKES. 

LIST  OF  CONTENTS  :  Introductory  and  Historical.  Genesis  rf 
Precious  Stones.  Physical  Properties.  The  Cutting  and  Polish- 
ing of  Gems.  Imitation  Gems  and  the  Artificial  Production  of 
Precious  Stones.  The  Diamond.  Fluor  Spar  and  the  Forms  of 
Silica.  Corundum,  including  Ruby  and  Sapphire.  Spinel  and 
Chrysoberyl.  The  Carbonates  and  the  Felspars.  The  Pyroxene 
and  Amphibole  Groups.  Beryl,  Cordierite,  Lapis  Lazuli  and  the 
Garnets.  Olivine,  Topaz,  Tourmaline  and  other  Silicates.  Phos- 
phates, Sulphates,  and  Carbon  Compounds. 

INTRODUCTION  TO  THE 

Chemistry  and  Physics  of  Building  Materials. 
By  ALAN  E.  MUNBY,  M.A.  365  Pages.  Illustrated. 

CONTENTS  :  Elementary  Science  :  Natural  Laws  and  Scientific  In- 
vestigations. Measurement  and  the  Properties  of  Matter.  Air 
and  Combustion.  Nature  and  Measurement  of  Heat  and  Its 
Effects  on  Materials.  Chemical  Signs  and  Calculations.  Water 
and  Its  Impurities.  Sulphur  and  the  Nature  of  Acids  and  Bases. 
Coal  and  Its  Products.  Outlines  of  Geology.  Building  Materials  : 
The  Constituents  of  Stones,  Clays  and  Cementing  Materials.  Clas- 
sification, Examination  and  Testing  of  Stones,  Brick  and  Other 
Clays.  Kiln  Reactions  and  the  Properties  of  Burnt  Clays.  Plasters 
and  Limes.  Cements.  Theories  upon  the  Setting  of  Plasters  and 
Hydraulic  Materials.  Artificial  Stone.  Oxychloride  Cement. 
Asphaite.  General  Properties  of  Metals.  Iron  and  Steel.  Other 
Metals  and  Alloys.  Timber.  Paints :  Oils,  Thinners  and  Varnishes ; 
Bases,  Pigments  and  Driers. 

Patents,  Designs  and  Trade  Marks  :  The  Law 
and  Commercial  Usage.  By  KENNETH  R.  SWAN, 
B.A.  (Oxon.),  of  the  Inner  Temple,  Barrister-at-Law. 
402  Pages. 

CONTENTS  :  Table  of  Cases  Cited— Part  I.— Letters  Patent.  Intro- 
duction. General.  Historical.  I.,  II.,  III.  Invention,  Novelty, 


THE     '  WESTMINSTER  "    SERIES 

Subject  Matter,  and  Utility  the  Essentials  of  Patentable  Invention. 
IV.  Specification.  V.  Construction  of  Specification.  VI.  Who 
May  Apply  for  a  Patent.  VII.  Application  and  Grant.  VIII. 
Opposition.  IX.  Patent  Rights.  Legal  Value.  Commercial 
Value.  X.  Amendment.  XI.  Infringement  of  Patent.  XII. 
Action  for  Infringement.  XIII.  Action  to  Restrain  Threats. 
XIV.  Negotiation  of  Patents  by  Sale  and  Licence.  XV.  Limita- 
tions on  Patent  Right.  XVI.  Revocation.  XVII.  Prolonga- 
tion. XVIII.  Miscellaneous.  XIX.  Foreign  Patents.  XX. 
Foreign  Patent  Laws  :  United  States  of  America.  Germany. 
France.  Table  of  Cost,  etc.,  of  Foreign  Patents.  APPENDIX  A. — 
i.  Table  of  Forms  and  Fees.  2.  Cost  of  Obtaining  a  British 
Patent.  3.  Convention  Countries.  Part  II. — Copyright  in 
Design.  Introduction.  I.  Registrable  Designs.  II.  Registra- 
tion. III.  Marking.  IV.  Infringement.  APPENDIX  B. — i. 
Table  of  Forms  and  Fees.  2.  Classification  of  Goods.  Part 
III. — Trade  Marks.  Introduction.  I.  Meaning  of  Trade  Mark. 
II.  Qualification  for  Registration.  III.  Restrictions  on  Regis- 
tration. IV.  Registration.  V.  Effect  of  Registration.  VI. 
Miscellaneous.  APPENDIX  C. — Table  of  Forms  and  Fees.  INDICES. 
i.  Patents.  2.  Designs.  3.  Trade  Marks. 

The  Book:  Its  History  and  Development.  By 
CYRIL  DAVENPORT,  V.D.,  F.S.A.  266  Pages.  With 
7  Plates  and  126  Figures  in  the  text. 

LIST  OF  CONTENTS  :  Early  Records.  Rolls,  Books  and  Book 
bindings.  Paper.  Printing.  Illustrations.  Miscellanea. 
Leathers.  The  Ornamentation  of  Leather  Bookbindings  without 
Gold.  The  Ornamentation  of  Leather  Bookbindings  with  Gold. 
Bibliography.  Index. 


The  Manufacture  of  Paper.  By  R.  W.  SINDALL,  F.C.S., 
Consulting  Chemist  to  the  Wood  Pulp  and  Paper  Trades  ; 
Lecturer  on  Paper-making  for  the  Hertfordshire  County 
Council,  the  Bucks  County  Council,  the  Printing  and 
Stationery  Trades  at  Exeter  Hall  (1903-4),  the  Institute 
of  Printers ;  Technical  Adviser  to  the  Government  of 
India,  1905.  275  Pages.  58  Illustrations. 

CONTENTS  :  Preface.  List  of  Illustrations.  Historical  Notice.  Cel- 
lulose and  Paper-making  Fibres.  The  Manufacture  of  Paper  from 
Rags,  Esparto  and  Straw.  Wood  Pulp  and  Wood  Pulp  Papers. 
Brown  Papers  and  Boards.  Special  kinds  of  Paper.  Chemicals 
used  in  Paper-making.  The  Process  of  "  Beating."  The  Dye- 
ing and  Colouring  of  Paper  Pulp.  Paper  Mill  Machinery.  The 
Deterioration  of  Paper.  Bibliography.  Index. 

(  7  ) 


THE   "WESTMINSTER"   SERIES 


Wood  Pulp  and  its  Applications*  By  C.  F.  CROSS, 
B.Sc.,  F.I.C.,  E.  J.  BEVAN,  F.I.C.,  and  R.  W.  SINDALL, 
F.C.S.  266  pages.  36  Illustrations. 

CONTENTS:  The  Structural  Elements  of  Wood.  Cellulose  as  a 
Chemical.  Sources  of  Supply.  Mechanical  Wood  Pulp.  Chemical 
Wood  Pulp.  The  Bleaching  of  Wood  Pulp.  News  and  Printings. 
Wood  Pulp  Boards.  Utilisation  of  Wood  Waste.  Testing  of 
Wood  Pulp  for  Moisture.  Wood  Pulp  and  the  Textile  Industries. 
Bibliography.  Index. 


Photography:  its  Principles  and  Applications. 
By  ALFRED  WATKINS,  F.R.P.S.  342  pages.  98  Illus- 
trations. 

CONTENTS  :  First  Principles.  Lenses.  Exposure  Influences.  Prac- 
tical Exposure.  Development  Influences.  Practical  Develop- 
ment. Cameras  and  Dark  Room.  Orthochromatic  Photography. 
Printing  Processes.  Hand  Camera  Work.  Enlarging  and  Slide 
Making.  Colour  Photography.  General  Applications.  Record 
Applications,  Science  Applications.  Plate  Speed  Testing.  Pro- 
cess Work.  Addenda.  Index. 


IN  PREPARATION. 

Commercial  Paints  and  Painting.  By  A.  S.  JENN- 
INGS, Hon.  Consulting  Examiner,  City  and  Guilds  of 
London  Institute. 

Brewing  and  Distilling*    By  JAMES  GRANT,  F.S.C 


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